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Annual report pursuant to Section 13 and 15(d)

PENSIONS AND OTHER POSTRETIREMENT BENEFITS PENSION AND OTHER POSTRETIREMENT BENEFITS (Schedule of Net Funded Status) (Details)

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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] Ìý Ìý Ìý
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items] Ìý Ìý Ìý
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) Ìý
Pension Plan [Member] | Salaried [Member] Ìý Ìý Ìý
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items] Ìý Ìý Ìý
Defined Benefit Plan, Plan Assets, Amount 293.7 249.8 Ìý
Defined Benefit Plan, Benefit Obligation (407.2) (340.8) Ìý
Funded (Unfunded) Status (113.5) (91.0) Ìý
Pension Plan [Member] | Iron Hourly [Member] Ìý Ìý Ìý
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items] Ìý Ìý Ìý
Defined Benefit Plan, Plan Assets, Amount 455.2 429.4 Ìý
Defined Benefit Plan, Benefit Obligation (607.3) (548.9) Ìý
Funded (Unfunded) Status (152.1) (119.5) Ìý
Pension Plan [Member] | Ore Mining [Member] Ìý Ìý Ìý
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items] Ìý Ìý Ìý
Defined Benefit Plan, Plan Assets, Amount Ìý 8.0 Ìý
Defined Benefit Plan, Benefit Obligation Ìý (10.7) Ìý
Funded (Unfunded) Status Ìý (2.7) Ìý
Pension Plan [Member] | SERP [Domain] Ìý Ìý Ìý
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items] Ìý Ìý Ìý
Defined Benefit Plan, Plan Assets, Amount 0.0 0.0 Ìý
Defined Benefit Plan, Benefit Obligation (6.6) (5.3) Ìý
Funded (Unfunded) Status (6.6) (5.3) Ìý
Other Postretirement Benefits Plan [Member] Ìý Ìý Ìý
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items] Ìý Ìý Ìý
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) Ìý
Other Postretirement Benefits Plan [Member] | Salaried [Member] Ìý Ìý Ìý
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items] Ìý Ìý Ìý
Defined Benefit Plan, Plan Assets, Amount 0.0 0.0 Ìý
Defined Benefit Plan, Benefit Obligation (37.7) (32.9) Ìý
Funded (Unfunded) Status (37.7) (32.9) Ìý
Other Postretirement Benefits Plan [Member] | Iron Hourly [Member] Ìý Ìý Ìý
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items] Ìý Ìý Ìý
Defined Benefit Plan, Plan Assets, Amount 259.7 240.2 Ìý
Defined Benefit Plan, Benefit Obligation (216.8) (209.0) Ìý
Funded (Unfunded) Status $ 42.9 $ 31.2 Ìý