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PENSIONS AND OTHER POSTRETIREMENT BENEFITS (Narrative) (Details) - USD ($) | 12 Months Ended
--- | ---
Dec. 31, 2016 | Dec. 31, 2015
Defined Benefit Plan Disclosure [Line Items] |  |
Minimum participants percentage | 15.00% |
Defined Benefit Plan, Accumulated Benefit Obligation | $ 922,000,000 | $ 898,900,000
Reserve for investment commitments | $ 37,900,000 |
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] |  |
Decrease in Projected Benefit Obligation due to Market Conditions | $ 22,000,000 |
Reduction in net periodic benefit expense | 8,200,000 |
Increase in Projected Benefit Obligation due to new mortality tables | $ 13,100,000 |
Percentage Increase in Projected Benefit Obligation due to new mortality tables | 1.40% |
Participant contributions | $ 0 | 0
Other Benefits [Member] |  |
Defined Benefit Plan Disclosure [Line Items] |  |
Decrease in Projected Benefit Obligation due to Market Conditions | 8,600,000 |
Reduction in net periodic benefit expense | 1,800,000 |
Increase in Projected Benefit Obligation due to new mortality tables | $ 4,900,000 |
Percentage Increase in Projected Benefit Obligation due to new mortality tables | 2.00% |
Participant contributions | $ 6,000,000 | $ 4,000,000
Tilden, Empire, Hibbing and United Taconite [Member] |  |
Defined Benefit Plan Disclosure [Line Items] |  |
Percentage of total workforce covered by labor contract | 80.00% |
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 |

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