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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% |

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- Definition For defined benefit pension plans, the actuarial present value of benefits (whether vested or nonvested) attributed by the pension benefit formula to employee service rendered before a specified date and based on employee service and compensation (if applicable) before that date. The accumulated benefit obligation differs from the projected benefit obligation in that it includes no assumption about future compensation levels. For plans with flat-benefit or nonpay-related pension benefit formulas, the accumulated benefit obligation and the projected benefit obligation are the same. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Glossary Accumulated Benefit Obligation -URI http://asc.fasb.org/extlink&oid=6503844 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 715 -SubTopic 20 -Section 50 -Paragraph 1 -Subparagraph (e) -URI http://asc.fasb.org/extlink&oid=58740216&loc=d3e1928-114920 + Details
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- Definition The amount of net gain (loss) recognized in net periodic benefit cost as a result of an event that significantly reduces the expected years of future service of present employees or eliminates for a significant number of employees the accrual of defined benefits for some or all of their future services. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Glossary Plan Curtailment -URI http://asc.fasb.org/extlink&oid=6520938 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 715 -SubTopic 20 -Section 50 -Paragraph 1 -Subparagraph (h)(7) -URI http://asc.fasb.org/extlink&oid=58740216&loc=d3e1928-114920 Reference 3: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 715 -SubTopic 30 -Section 15 -Paragraph 6 -Subparagraph (a)(2) -URI http://asc.fasb.org/extlink&oid=7577095&loc=d3e8001-114927 + Details
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- Definition Amount of net gain (loss) recognized in net periodic benefit cost due to settlements and curtailments. Curtailments result from an event that significantly reduces the expected years of future service of present employees or eliminates for a significant number of employees the accrual of defined benefits for some or all of their future services. Settlements result from an irrevocable action that relieves the employer (or the plan) of primary responsibility for a benefit obligation and eliminates significant risks related to the obligation and the assets used to effect the settlement. Examples of transactions that constitute a settlement include, but are not limited to, lump-sum cash payments to plan participants in exchange for their rights to receive specified benefits and purchasing nonparticipating annuity contracts to cover vested benefits. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 715 -SubTopic 20 -Section 50 -Paragraph 1 -Subparagraph (h)(7) -URI http://asc.fasb.org/extlink&oid=58740216&loc=d3e1928-114920 + Details
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