Annual report pursuant to Section 13 and 15(d)

PENSIONS AND OTHER POSTRETIREMENT BENEFITS (Narrative) (Details)

v2.4.0.6
PENSIONS AND OTHER POSTRETIREMENT BENEFITS (Narrative) (Details) (USD $)
 12 Months Ended 12 Months Ended 12 Months Ended 12 Months Ended 12 Months Ended
Dec. 31, 2012
employee
Dec. 31, 2011
Dec. 31, 2012
Sei Opportunity Collective Fund [Member]
Dec. 31, 2012
Sei Special Situations Fund [Member]
Dec. 31, 2012
Structured Finance [Member]
Dec. 31, 2012
Real Estate [Member]
Dec. 31, 2012
Pension Benefits [Member]
Dec. 31, 2011
Pension Benefits [Member]
Dec. 31, 2010
Pension Benefits [Member]
Dec. 31, 2012
Other Benefits [Member]
Dec. 31, 2011
Other Benefits [Member]
Dec. 31, 2012
Northshore [Member]
Dec. 31, 2012
North American Coal [Member]
Dec. 31, 2011
North American Coal [Member]
Dec. 31, 2010
North American Coal [Member]
Dec. 31, 2012
Defined Benefit Postretirement Health Coverage [Member]
United States Postretirement Benefit Plans of US Entity, Defined Benefit [Member]
Prior To Age65 [Member]
Dec. 31, 2012
Defined Benefit Postretirement Health Coverage [Member]
United States Postretirement Benefit Plans of US Entity, Defined Benefit [Member]
After Age65 [Member]
Dec. 31, 2012
Defined Benefit Postretirement Health Coverage [Member]
Northshore [Member]
After Age65 [Member]
Dec. 31, 2012
Minimum [Member]
Dec. 31, 2012
Minimum [Member]
Defined Benefit Postretirement Health Coverage [Member]
Northshore [Member]
Prior To Age65 [Member]
Dec. 31, 2012
Maximum [Member]
Dec. 31, 2011
Maximum [Member]
Dec. 31, 2012
Maximum [Member]
Defined Benefit Postretirement Health Coverage [Member]
Northshore [Member]
Prior To Age65 [Member]
Dec. 31, 2012
United States [Member]
Pension Benefits [Member]
security
Dec. 31, 2011
United States [Member]
Pension Benefits [Member]
Dec. 31, 2012
United States [Member]
Other Benefits [Member]
Dec. 31, 2011
United States [Member]
Other Benefits [Member]
Dec. 31, 2012
Canada [Member]
Pension Benefits [Member]
security
Dec. 31, 2011
Canada [Member]
Pension Benefits [Member]
Dec. 31, 2012
Canada [Member]
Other Benefits [Member]
Dec. 31, 2011
Canada [Member]
Other Benefits [Member]
Feb. 28, 2010
Wabush [Member]
Defined Benefit Plan Disclosure [Line Items]                                                                
Remaining interest percentage prior to acquisition                       100.00%                                       73.20%
Term of labor contracts with USW 37 months                                                              
Number of USW employees covered by labor contracts 2,400                                                              
Percentage of total workforce covered by labor contract 32.00%                       11.00%                                      
Annual limit on medical coverage for each participant                               $ 7,000 $ 3,000 $ 2,000   $ 4,020     $ 4,500                  
Minimum participants percentage 15.00%                                                              
Contribution rate per hour worked                         8.10 6.50 6.42                                  
Annual costs for defined contribution plan             6,700,000 5,700,000 4,200,000       14,900,000 9,500,000 10,300,000                                  
Funding of bargaining unit pension plans and VEBAs 65,700,000                                                              
Accumulated benefit obligation for defined benefit pension plans 1,204,700,000 1,114,700,000                                                            
Discount rate                                     3.54%     3.80%   3.70% 4.28% 3.70% 4.28% 3.75% 4.00% 4.00% 4.25%  
Projected yield, number of bonds used                                               506       240        
Maturity period of security 10 years                                                              
Expected period of long-term return on assets                                     3 years   5 years                      
Redemption request notice period, days     65 days 95 days 90 days 45 days                                                    
Withdrawal request notice period, days           65 days                                                    
Reserve for investment commitments 10,700,000                                                              
Participant contributions             $ 0 $ 0   $ 4,600,000 $ 4,600,000