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PENSIONS AND OTHER POSTRETIREMENT BENEFITS PENSION AND OTHER POSTRETIREMENT BENEFITS (Schedule of Net Funded Status) (Details) - USD ($) $ in Millions | Dec. 31, 2019 | Dec. 31, 2018 | Dec. 31, 2017
--- | --- | --- | ---
Pension Plan [Member] |  |  |
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items] |  |  |
Defined Benefit Plan, Plan Assets, Amount | $ 748.9 | $ 687.2 | $ 749.8
Defined Benefit Plan, Benefit Obligation | (1,021.1) | (905.7) | (973.1)
Funded (Unfunded) Status | (272.2) | (218.5) |
Pension Plan [Member] | Salaried [Member] |  |  |
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items] |  |  |
Defined Benefit Plan, Plan Assets, Amount | 293.7 | 249.8 |
Defined Benefit Plan, Benefit Obligation | (407.2) | (340.8) |
Funded (Unfunded) Status | (113.5) | (91.0) |
Pension Plan [Member] | Iron Hourly [Member] |  |  |
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items] |  |  |
Defined Benefit Plan, Plan Assets, Amount | 455.2 | 429.4 |
Defined Benefit Plan, Benefit Obligation | (607.3) | (548.9) |
Funded (Unfunded) Status | (152.1) | (119.5) |
Pension Plan [Member] | Ore Mining [Member] |  |  |
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items] |  |  |
Defined Benefit Plan, Plan Assets, Amount |  | 8.0 |
Defined Benefit Plan, Benefit Obligation |  | (10.7) |
Funded (Unfunded) Status |  | (2.7) |
Pension Plan [Member] | SERP [Domain] |  |  |
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items] |  |  |
Defined Benefit Plan, Plan Assets, Amount | 0.0 | 0.0 |
Defined Benefit Plan, Benefit Obligation | (6.6) | (5.3) |
Funded (Unfunded) Status | (6.6) | (5.3) |
Other Postretirement Benefits Plan [Member] |  |  |
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items] |  |  |
Defined Benefit Plan, Plan Assets, Amount | 259.7 | 240.2 | 262.5
Defined Benefit Plan, Benefit Obligation | (254.5) | (241.9) | $ (265.9)
Funded (Unfunded) Status | 5.2 | (1.7) |
Other Postretirement Benefits Plan [Member] | Salaried [Member] |  |  |
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items] |  |  |
Defined Benefit Plan, Plan Assets, Amount | 0.0 | 0.0 |
Defined Benefit Plan, Benefit Obligation | (37.7) | (32.9) |
Funded (Unfunded) Status | (37.7) | (32.9) |
Other Postretirement Benefits Plan [Member] | Iron Hourly [Member] |  |  |
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items] |  |  |
Defined Benefit Plan, Plan Assets, Amount | 259.7 | 240.2 |
Defined Benefit Plan, Benefit Obligation | (216.8) | (209.0) |
Funded (Unfunded) Status | $ 42.9 | $ 31.2 |

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