PENSIONS AND OTHER POSTRETIREMENT BENEFITS (Narrative) (Details) (USD $)	12 Months Ended
	Dec. 31, 2013	Dec. 31, 2012	Dec. 31, 2011
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	$ 1,091,400,000	$ 1,204,700,000
Maturity period of security	10 years
Reserve for investment commitments	8,700,000
Sei Special Situations Fund [Member]
Defined Benefit Plan Disclosure [Line Items]
Redemption request notice period, days	95 days
Structured Finance [Member]
Defined Benefit Plan Disclosure [Line Items]
Redemption request notice period, days	90 days
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]
Defined Benefit Plan, Net Periodic Benefit Cost	52,100,000	55,200,000	37,800,000
Annual costs for defined contribution plan	6,800,000	6,700,000	5,700,000
Participant contributions	0	0
Other Postretirement Benefit Expense	17,400,000	28,100,000	26,800,000
Pension and Other Postretirement Benefit Expense	76,300,000	90,000,000	70,300,000
Other Benefits [Member]
Defined Benefit Plan Disclosure [Line Items]
Defined Benefit Plan, Net Periodic Benefit Cost	17,400,000	28,100,000	26,800,000
Participant contributions	5,600,000	4,600,000
Defined Benefit Postretirement Health Coverage [Member] | United States Postretirement Benefit Plans of US Entity, Defined Benefit [Member] | Prior To Age65 [Member]
Defined Benefit Plan Disclosure [Line Items]
Annual limit on medical coverage for each participant	7,000
Defined Benefit Postretirement Health Coverage [Member] | United States Postretirement Benefit Plans of US Entity, Defined Benefit [Member] | After Age65 [Member]
Defined Benefit Plan Disclosure [Line Items]
Annual limit on medical coverage for each participant	3,000
Defined Benefit Postretirement Health Coverage [Member] | Northshore [Member] | After Age65 [Member]
Defined Benefit Plan Disclosure [Line Items]
Annual limit on medical coverage for each participant	2,000
Minimum [Member]
Defined Benefit Plan Disclosure [Line Items]
Discount rate	4.36%
Expected period of long-term return on assets	3 years
Minimum [Member] | Defined Benefit Postretirement Health Coverage [Member] | Northshore [Member] | Prior To Age65 [Member]
Defined Benefit Plan Disclosure [Line Items]
Annual limit on medical coverage for each participant	4,020
Maximum [Member]
Defined Benefit Plan Disclosure [Line Items]
Discount rate	4.66%
Expected period of long-term return on assets	5 years
Maximum [Member] | Defined Benefit Postretirement Health Coverage [Member] | Northshore [Member] | Prior To Age65 [Member]
Defined Benefit Plan Disclosure [Line Items]
Annual limit on medical coverage for each participant	4,500
United States [Member] | Pension Benefits [Member]
Defined Benefit Plan Disclosure [Line Items]
Discount rate	4.57%	3.70%
Projected yield, number of bonds used	494
United States [Member] | Other Benefits [Member]
Defined Benefit Plan Disclosure [Line Items]
Discount rate	4.57%	3.70%
Canada [Member] | Pension Benefits [Member]
Defined Benefit Plan Disclosure [Line Items]
Discount rate	4.50%	3.75%
Projected yield, number of bonds used	334
Canada [Member] | Other Benefits [Member]
Defined Benefit Plan Disclosure [Line Items]
Discount rate	4.75%	4.00%
Tilden, Empire, Hibbing and United Taconite [Member]
Defined Benefit Plan Disclosure [Line Items]
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%
Funding of bargaining unit pension plans and VEBAs	65,700,000
Bloom Lake [Member]
Defined Benefit Plan Disclosure [Line Items]
Term of labor contracts with USW	3 years
Pointe Noire [Member]
Defined Benefit Plan Disclosure [Line Items]
Term of labor contracts with USW	6 years
Increased in Cost of Employment over Life of Union Contract	26.00%
Pinnacle Complex and Oak Grove [Member]
Defined Benefit Plan Disclosure [Line Items]
Number of USW employees covered by labor contracts	800
Percentage of total workforce covered by labor contract	11.00%
Contribution rate per hour worked	8.10	8.10	6.50
Annual costs for defined contribution plan	14,900,000	14,900,000	9,500,000
Underfunded Liability Estimate	$ 342,000,000