Annual report pursuant to Section 13 and 15(d)

PENSIONS AND OTHER POSTRETIREMENT BENEFITS PENSION AND OTHER POSTRETIREMENT BENEFITS (Schedule of Net Funded Status) (Details)

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PENSIONS AND OTHER POSTRETIREMENT BENEFITS PENSION AND OTHER POSTRETIREMENT BENEFITS (Schedule of Net Funded Status) (Details) - USD ($)
$ in Millions
Dec. 31, 2019
Dec. 31, 2018
Dec. 31, 2017
Pension Plan [Member]      
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]      
Defined Benefit Plan, Plan Assets, Amount $ 748.9 $ 687.2 $ 749.8
Defined Benefit Plan, Benefit Obligation (1,021.1) (905.7) (973.1)
Funded (Unfunded) Status (272.2) (218.5)  
Pension Plan [Member] | Salaried [Member]      
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]      
Defined Benefit Plan, Plan Assets, Amount 293.7 249.8  
Defined Benefit Plan, Benefit Obligation (407.2) (340.8)  
Funded (Unfunded) Status (113.5) (91.0)  
Pension Plan [Member] | Iron Hourly [Member]      
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]      
Defined Benefit Plan, Plan Assets, Amount 455.2 429.4  
Defined Benefit Plan, Benefit Obligation (607.3) (548.9)  
Funded (Unfunded) Status (152.1) (119.5)  
Pension Plan [Member] | Ore Mining [Member]      
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]      
Defined Benefit Plan, Plan Assets, Amount   8.0  
Defined Benefit Plan, Benefit Obligation   (10.7)  
Funded (Unfunded) Status   (2.7)  
Pension Plan [Member] | SERP [Domain]      
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]      
Defined Benefit Plan, Plan Assets, Amount 0.0 0.0  
Defined Benefit Plan, Benefit Obligation (6.6) (5.3)  
Funded (Unfunded) Status (6.6) (5.3)  
Other Postretirement Benefits Plan [Member]      
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]      
Defined Benefit Plan, Plan Assets, Amount 259.7 240.2 262.5
Defined Benefit Plan, Benefit Obligation (254.5) (241.9) $ (265.9)
Funded (Unfunded) Status 5.2 (1.7)  
Other Postretirement Benefits Plan [Member] | Salaried [Member]      
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]      
Defined Benefit Plan, Plan Assets, Amount 0.0 0.0  
Defined Benefit Plan, Benefit Obligation (37.7) (32.9)  
Funded (Unfunded) Status (37.7) (32.9)  
Other Postretirement Benefits Plan [Member] | Iron Hourly [Member]      
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]      
Defined Benefit Plan, Plan Assets, Amount 259.7 240.2  
Defined Benefit Plan, Benefit Obligation (216.8) (209.0)  
Funded (Unfunded) Status $ 42.9 $ 31.2