Annual report pursuant to Section 13 and 15(d)

PENSIONS AND OTHER POSTRETIREMENT BENEFITS (Narrative) (Details)

v3.3.1.900
PENSIONS AND OTHER POSTRETIREMENT BENEFITS (Narrative) (Details)
 12 Months Ended
Dec. 31, 2015
USD ($)
employee
Dec. 31, 2014
USD ($)
Dec. 31, 2013
USD ($)
Defined Benefit Plan Disclosure [Line Items]      
Current Fiscal Year End Date --12-31    
Minimum participants percentage 15.00%    
Accumulated benefit obligation for defined benefit pension plans $ 898,900,000 $ 980,600,000  
Reserve for investment commitments $ 48,100,000    
Structured Finance [Member]      
Defined Benefit Plan Disclosure [Line Items]      
Redemption request notice period, days 90 days    
Redemption Request $ 8,000,000    
Real Estate [Member]      
Defined Benefit Plan Disclosure [Line Items]      
Redemption request notice period, days 45 days    
Withdrawal request notice period, days 65 days    
Pension Benefits [Member]      
Defined Benefit Plan Disclosure [Line Items]      
Increase in Projected Benefit Obligation due to new mortality tables $ 15,100,000    
Percentage Increase in Projected Benefit Obligation due to new mortality tables 1.50%    
Defined Benefit Plan, Net Periodic Benefit Cost $ 23,900,000 26,200,000 $ 46,800,000
Annual costs for defined contribution plan 3,600,000 4,400,000 5,000,000
Decrease in Projected Benefit Obligation due to Market Conditions 31,500,000    
Decrease in Projected Benefit Obligation due to Change in Percentiles 8,300,000    
Participant contributions 0 0  
Other Postretirement Benefit Expense 4,400,000 (2,500,000) 3,200,000
Pension and Other Postretirement Benefit Expense 31,900,000 28,100,000 55,000,000
Defined Benefit Plan, Recognized Net Gain (Loss) Due to Curtailments (1,200,000) (1,500,000)  
Defined Benefit Plan, Recognized Net Gain (Loss) Due to Settlements and Curtailments (200,000) (1,400,000) 0
Other Benefits [Member]      
Defined Benefit Plan Disclosure [Line Items]      
Increase in Projected Benefit Obligation due to new mortality tables $ 7,900,000    
Percentage Increase in Projected Benefit Obligation due to new mortality tables 2.00%    
Defined Benefit Plan, Net Periodic Benefit Cost $ 4,400,000 (2,500,000) 3,200,000
Decrease in Projected Benefit Obligation due to Market Conditions 13,600,000    
Decrease in Projected Benefit Obligation due to Change in Percentiles 2,700,000    
Participant contributions 4,000,000 4,800,000  
Defined Benefit Plan, Recognized Net Gain (Loss) Due to Curtailments 0 0  
Defined Benefit Plan, Recognized Net Gain (Loss) Due to Settlements and Curtailments $ 0 $ 0 $ 0
United States [Member] | Pension Benefits [Member]      
Defined Benefit Plan Disclosure [Line Items]      
Discount rate   3.83%  
Tilden, Empire, Hibbing and United Taconite [Member]      
Defined Benefit Plan Disclosure [Line Items]      
Number of USW employees covered by labor contracts | employee 2,000    
Percentage of total workforce covered by labor contract 81.00%    
Prior To Age65 [Member] | Defined Benefit Postretirement Health Coverage [Member] | United States [Member]      
Defined Benefit Plan Disclosure [Line Items]      
Annual limit on medical coverage for each participant $ 7,000    
Prior To Age65 [Member] | Minimum [Member] | Defined Benefit Postretirement Health Coverage [Member] | Northshore [Member]      
Defined Benefit Plan Disclosure [Line Items]      
Annual limit on medical coverage for each participant 4,020    
Prior To Age65 [Member] | Maximum [Member] | Defined Benefit Postretirement Health Coverage [Member] | Northshore [Member]      
Defined Benefit Plan Disclosure [Line Items]      
Annual limit on medical coverage for each participant $ 4,500    
Hourly [Member] | Other Benefits [Member]      
Defined Benefit Plan Disclosure [Line Items]      
Discount rate 4.32% 3.83%  
Supplemental Executive Retirement Plan S E R P [Member] | Pension Benefits [Member]      
Defined Benefit Plan Disclosure [Line Items]      
Discount rate 4.22% 3.83%  
Ore Mining [Member] | Pension Benefits [Member]      
Defined Benefit Plan Disclosure [Line Items]      
Discount rate 4.28% 3.83%  
Iron Hourly [Member] | Pension Benefits [Member]      
Defined Benefit Plan Disclosure [Line Items]      
Discount rate 4.27% 3.83%  
Salaried [Member] | Pension Benefits [Member]      
Defined Benefit Plan Disclosure [Line Items]      
Discount rate 4.12% 3.83%  
Salaried [Member] | Other Benefits [Member]      
Defined Benefit Plan Disclosure [Line Items]      
Discount rate 4.22% 3.83%