Annual report pursuant to Section 13 and 15(d)

PENSIONS AND OTHER POSTRETIREMENT BENEFITS (Benefit Obligations, Fair Value of Plan Assets, and Net Funded Status) (Details)

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PENSIONS AND OTHER POSTRETIREMENT BENEFITS (Benefit Obligations, Fair Value of Plan Assets, and Net Funded Status) (Details) - USD ($)
$ in Millions
 12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Dec. 31, 2017
Dec. 31, 2016
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]          
Fair value of plan assets — beginning of year $ 202.4 $ 193.4      
Employer contributions 2.1 1.1      
Fair value of plan assets — end of year 215.6 202.4 $ 193.4    
Funded status at December 31:          
Defined Benefit Plan, Fair Value of Plan Assets 202.4 193.4 193.4 $ 215.6 $ 202.4
Amounts recognized in Statements of Financial Position:          
Assets, Current       1,544.6 824.6
TOTAL POSTEMPLOYMENT BENEFIT LIABILITIES       (257.7) (280.5)
Pension Benefits [Member]          
Defined Benefit Plan, Change in Benefit Obligation [Roll Forward]          
Benefit obligations — beginning of year (931.6) (910.8)      
Service cost (excluding expenses) 17.1 17.6 22.7    
Interest cost 30.5 30.3 37.7    
Plan amendments 0.0 5.7      
Actuarial loss (22.3) (21.1) (20.8)    
Defined Benefit Plan, Change in Actuarial (gain) loss 54.6 38.1      
Defined Benefit Plan, Benefit Obligation, Benefits Paid 60.7 70.9      
Participant contributions 0.0 0.0      
Federal subsidy on benefits paid 0.0 0.0      
Benefit obligations — end of year (973.1) (931.6) (910.8)    
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]          
Fair value of plan assets — beginning of year 685.8 700.6      
Actual return on plan assets 100.2 54.8      
Participant contributions 0.0 0.0      
Employer contributions 24.4 1.2      
Assets Transferred to (from) Plan 0.1 0.1      
Benefits paid (60.7) (70.9)      
Fair value of plan assets — end of year 749.8 685.8 700.6    
Funded status at December 31:          
Defined Benefit Plan, Fair Value of Plan Assets 685.8 700.6 700.6 749.8 685.8
Defined Benefit Plan, Benefit Obligation (931.6) (910.8) (910.8) (973.1) (931.6)
Defined Benefit Plan, Funded (Unfunded) Status of Plan       (223.3) (245.8)
Total amount recognized       (223.3) (245.8)
Amounts recognized in Statements of Financial Position:          
Assets, Current       0.0 0.0
Current liabilities       (0.5) (0.1)
TOTAL POSTEMPLOYMENT BENEFIT LIABILITIES       (222.8) (245.7)
Total amount recognized       (223.3) (245.8)
Amounts recognized in accumulated other comprehensive income:          
Prior service cost       8.8 11.0
Net actuarial loss       318.7 315.9
Net amount recognized       327.5 326.9
The estimated amounts that will be amortized from accumulated other comprehensive income into net periodic benefit cost:          
Net actuarial loss       21.1  
Prior service cost       2.2  
Net amount recognized       23.3  
Other Benefits [Member]          
Defined Benefit Plan, Change in Benefit Obligation [Roll Forward]          
Benefit obligations — beginning of year (264.6) (266.0)      
Service cost (excluding expenses) 1.8 1.7 6.4    
Interest cost 8.3 9.1 13.4    
Plan amendments 0.0 9.8      
Actuarial loss (4.5) (6.0) (6.6)    
Defined Benefit Plan, Change in Actuarial (gain) loss 7.4 (7.2)      
Defined Benefit Plan, Benefit Obligation, Benefits Paid 21.4 21.3      
Participant contributions 4.6 6.0      
Federal subsidy on benefits paid 0.6 0.5      
Benefit obligations — end of year (265.9) (264.6) (266.0)    
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]          
Fair value of plan assets — beginning of year 253.0 250.6      
Actual return on plan assets 24.2 16.0      
Participant contributions 0.3 0.5      
Employer contributions 1.7 1.7      
Assets Transferred to (from) Plan 0.0 0.0      
Benefits paid (16.7) (15.8)      
Fair value of plan assets — end of year 262.5 253.0 250.6    
Funded status at December 31:          
Defined Benefit Plan, Fair Value of Plan Assets 253.0 250.6 250.6 262.5 253.0
Defined Benefit Plan, Benefit Obligation (264.6) (266.0) $ (266.0) (265.9) (264.6)
Defined Benefit Plan, Funded (Unfunded) Status of Plan       (3.4) (11.6)
Total amount recognized       (3.4) (11.6)
Amounts recognized in Statements of Financial Position:          
Assets, Current         27.3
Assets, Noncurrent       35.4  
Current liabilities       (3.9) (4.1)
TOTAL POSTEMPLOYMENT BENEFIT LIABILITIES       (34.9) (34.8)
Total amount recognized       (3.4) (11.6)
Amounts recognized in accumulated other comprehensive income:          
Prior service cost       (25.6) (26.9)
Net actuarial loss       88.3 87.0
Net amount recognized       62.7 60.1
The estimated amounts that will be amortized from accumulated other comprehensive income into net periodic benefit cost:          
Net actuarial loss       4.9  
Prior service cost       (3.0)  
Net amount recognized       1.9  
Salaried Employees [Member] | Pension Benefits [Member]          
Defined Benefit Plan, Change in Benefit Obligation [Roll Forward]          
Benefit obligations — beginning of year (351.9)        
Benefit obligations — end of year (368.0) (351.9)      
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]          
Fair value of plan assets — beginning of year 242.9        
Fair value of plan assets — end of year 269.4 242.9      
Funded status at December 31:          
Defined Benefit Plan, Fair Value of Plan Assets 242.9 242.9   269.4 242.9
Defined Benefit Plan, Benefit Obligation (351.9) (351.9)   (368.0) (351.9)
Defined Benefit Plan, Funded (Unfunded) Status of Plan       (98.6) (109.0)
Salaried Employees [Member] | Other Benefits [Member]          
Defined Benefit Plan, Change in Benefit Obligation [Roll Forward]          
Benefit obligations — beginning of year (37.6)        
Benefit obligations — end of year (37.7) (37.6)      
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]          
Fair value of plan assets — beginning of year 0.0        
Fair value of plan assets — end of year 0.0 0.0      
Funded status at December 31:          
Defined Benefit Plan, Fair Value of Plan Assets 0.0 0.0   0.0 0.0
Defined Benefit Plan, Benefit Obligation (37.6) (37.6)   (37.7) (37.6)
Defined Benefit Plan, Funded (Unfunded) Status of Plan       (37.7) (37.6)
Hourly Employees [Member] | Pension Benefits [Member]          
Defined Benefit Plan, Change in Benefit Obligation [Roll Forward]          
Benefit obligations — beginning of year (565.6)        
Benefit obligations — end of year (590.0) (565.6)      
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]          
Fair value of plan assets — beginning of year 436.9        
Fair value of plan assets — end of year 473.0 436.9      
Funded status at December 31:          
Defined Benefit Plan, Fair Value of Plan Assets 436.9 436.9   473.0 436.9
Defined Benefit Plan, Benefit Obligation (565.6) (565.6)   (590.0) (565.6)
Defined Benefit Plan, Funded (Unfunded) Status of Plan       (117.0) (128.7)
Hourly Employees [Member] | Other Benefits [Member]          
Defined Benefit Plan, Change in Benefit Obligation [Roll Forward]          
Benefit obligations — beginning of year (227.0)        
Benefit obligations — end of year (228.2) (227.0)      
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]          
Fair value of plan assets — beginning of year 253.0        
Fair value of plan assets — end of year 262.5 253.0      
Funded status at December 31:          
Defined Benefit Plan, Fair Value of Plan Assets 253.0 253.0   262.5 253.0
Defined Benefit Plan, Benefit Obligation (227.0) (227.0)   (228.2) (227.0)
Defined Benefit Plan, Funded (Unfunded) Status of Plan       34.3 26.0
Mining Employees [Member] | Pension Benefits [Member]          
Defined Benefit Plan, Change in Benefit Obligation [Roll Forward]          
Benefit obligations — beginning of year (10.0)        
Benefit obligations — end of year (10.3) (10.0)      
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]          
Fair value of plan assets — beginning of year 6.0        
Fair value of plan assets — end of year 7.4 6.0      
Funded status at December 31:          
Defined Benefit Plan, Fair Value of Plan Assets 6.0 6.0   7.4 6.0
Defined Benefit Plan, Benefit Obligation (10.0) (10.0)   (10.3) (10.0)
Defined Benefit Plan, Funded (Unfunded) Status of Plan       (2.9) (4.0)
Supplemental Executive Retirement Plan S E R P [Member] | Pension Benefits [Member]          
Defined Benefit Plan, Change in Benefit Obligation [Roll Forward]          
Benefit obligations — beginning of year (4.1)        
Benefit obligations — end of year (4.8) (4.1)      
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]          
Fair value of plan assets — beginning of year 0.0        
Fair value of plan assets — end of year 0.0 0.0      
Funded status at December 31:          
Defined Benefit Plan, Fair Value of Plan Assets 0.0 0.0   0.0 0.0
Defined Benefit Plan, Benefit Obligation $ (4.1) $ (4.1)   (4.8) (4.1)
Defined Benefit Plan, Funded (Unfunded) Status of Plan       $ (4.8) $ (4.1)