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Annual report pursuant to Section 13 and 15(d)

PENSIONS AND OTHER POSTRETIREMENT BENEFITS (Benefit Obligations, Fair Value of Plan Assets, and Net Funded Status) (Details)

v3.10.0.1
PENSIONS AND OTHER POSTRETIREMENT BENEFITS (Benefit Obligations, Fair Value of Plan Assets, and Net Funded Status) (Details) - USD ($)
$ in Millions
 12 Months Ended
Dec. 31, 2018
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2018
Jan. 01, 2018
Dec. 31, 2017
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward] Ìý Ìý Ìý Ìý Ìý Ìý
Fair value of plan assets — beginning of year $ 215.6 $ 202.4 Ìý Ìý Ìý Ìý
Employer contributions 3.8 2.1 Ìý Ìý Ìý Ìý
Fair value of plan assets — end of year 229.1 215.6 $ 202.4 Ìý Ìý Ìý
Funded status at December 31: Ìý Ìý Ìý Ìý Ìý Ìý
Defined Benefit Plan, Plan Assets, Amount 215.6 202.4 202.4 $ 229.1 Ìý $ 215.6
Amounts recognized in Statements of Financial Position: Ìý Ìý Ìý Ìý Ìý Ìý
TOTAL POSTEMPLOYMENT BENEFIT LIABILITIES Ìý Ìý Ìý (248.7) $ (257.7) (257.7)
Pension Benefits [Member] Ìý Ìý Ìý Ìý Ìý Ìý
Defined Benefit Plan, Change in Benefit Obligation [Roll Forward] Ìý Ìý Ìý Ìý Ìý Ìý
Benefit obligations — beginning of year (973.1) (931.6) Ìý Ìý Ìý Ìý
Service cost (excluding expenses) 18.7 17.1 17.6 Ìý Ìý Ìý
Interest cost 30.3 30.5 30.3 Ìý Ìý Ìý
Plan amendments 2.2 0.0 Ìý Ìý Ìý Ìý
Defined Benefit Plan, Benefit Obligation, (Increase) Decrease for Curtailment (0.9) 0.0 Ìý Ìý Ìý Ìý
Defined Benefit Plan, Benefit Obligation, Actuarial Gain (Loss) (57.0) 54.6 Ìý Ìý Ìý Ìý
Defined Benefit Plan, Benefit Obligation, Benefits Paid 60.7 60.7 Ìý Ìý Ìý Ìý
Participant contributions 0.0 0.0 Ìý Ìý Ìý Ìý
Federal subsidy on benefits paid 0.0 0.0 Ìý Ìý Ìý Ìý
Benefit obligations — end of year (905.7) (973.1) (931.6) Ìý Ìý Ìý
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward] Ìý Ìý Ìý Ìý Ìý Ìý
Fair value of plan assets — beginning of year 749.8 685.8 Ìý Ìý Ìý Ìý
Actual return on plan assets (29.6) 100.2 Ìý Ìý Ìý Ìý
Participant contributions 0.0 0.0 Ìý Ìý Ìý Ìý
Employer contributions 27.6 24.4 Ìý Ìý Ìý Ìý
Assets Transferred to (from) Plan 0.1 0.1 Ìý Ìý Ìý Ìý
Benefits paid (60.7) (60.7) Ìý Ìý Ìý Ìý
Fair value of plan assets — end of year 687.2 749.8 685.8 Ìý Ìý Ìý
Funded status at December 31: Ìý Ìý Ìý Ìý Ìý Ìý
Defined Benefit Plan, Plan Assets, Amount 749.8 685.8 685.8 687.2 Ìý 749.8
Defined Benefit Plan, Benefit Obligation (973.1) (931.6) (931.6) (905.7) Ìý (973.1)
Defined Benefit Plan, Funded (Unfunded) Status of Plan Ìý Ìý Ìý (218.5) Ìý (223.3)
Total amount recognized Ìý Ìý Ìý (218.5) Ìý (223.3)
Amounts recognized in Statements of Financial Position: Ìý Ìý Ìý Ìý Ìý Ìý
Assets, Noncurrent Ìý Ìý Ìý 0.0 Ìý 0.0
Current liabilities Ìý Ìý Ìý (0.1) Ìý (0.5)
TOTAL POSTEMPLOYMENT BENEFIT LIABILITIES Ìý Ìý Ìý (218.4) Ìý (222.8)
Total amount recognized Ìý Ìý Ìý (218.5) Ìý (223.3)
Amounts recognized in accumulated other comprehensive income: Ìý Ìý Ìý Ìý Ìý Ìý
Prior service cost Ìý Ìý Ìý 8.5 Ìý 8.8
Net actuarial loss Ìý Ìý Ìý 330.1 Ìý 318.7
Net amount recognized Ìý Ìý Ìý 338.6 Ìý 327.5
Other Benefits [Member] Ìý Ìý Ìý Ìý Ìý Ìý
Defined Benefit Plan, Change in Benefit Obligation [Roll Forward] Ìý Ìý Ìý Ìý Ìý Ìý
Benefit obligations — beginning of year (265.9) (264.6) Ìý Ìý Ìý Ìý
Service cost (excluding expenses) 2.2 1.8 1.7 Ìý Ìý Ìý
Interest cost 8.3 8.3 9.1 Ìý Ìý Ìý
Plan amendments 12.8 0.0 Ìý Ìý Ìý Ìý
Defined Benefit Plan, Benefit Obligation, (Increase) Decrease for Curtailment 0.0 0.0 Ìý Ìý Ìý Ìý
Defined Benefit Plan, Benefit Obligation, Actuarial Gain (Loss) (29.4) 7.4 Ìý Ìý Ìý Ìý
Defined Benefit Plan, Benefit Obligation, Benefits Paid 24.4 21.4 Ìý Ìý Ìý Ìý
Participant contributions 5.6 4.6 Ìý Ìý Ìý Ìý
Federal subsidy on benefits paid 0.9 0.6 Ìý Ìý Ìý Ìý
Benefit obligations — end of year (241.9) (265.9) (264.6) Ìý Ìý Ìý
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward] Ìý Ìý Ìý Ìý Ìý Ìý
Fair value of plan assets — beginning of year 262.5 253.0 Ìý Ìý Ìý Ìý
Actual return on plan assets (8.2) 24.2 Ìý Ìý Ìý Ìý
Participant contributions 0.5 0.3 Ìý Ìý Ìý Ìý
Employer contributions 3.0 1.7 Ìý Ìý Ìý Ìý
Assets Transferred to (from) Plan 0.0 0.0 Ìý Ìý Ìý Ìý
Benefits paid (17.6) (16.7) Ìý Ìý Ìý Ìý
Fair value of plan assets — end of year 240.2 262.5 253.0 Ìý Ìý Ìý
Funded status at December 31: Ìý Ìý Ìý Ìý Ìý Ìý
Defined Benefit Plan, Plan Assets, Amount 262.5 253.0 253.0 240.2 Ìý 262.5
Defined Benefit Plan, Benefit Obligation (265.9) (264.6) $ (264.6) (241.9) Ìý (265.9)
Defined Benefit Plan, Funded (Unfunded) Status of Plan Ìý Ìý Ìý (1.7) Ìý (3.4)
Total amount recognized Ìý Ìý Ìý (1.7) Ìý (3.4)
Amounts recognized in Statements of Financial Position: Ìý Ìý Ìý Ìý Ìý Ìý
Assets, Noncurrent Ìý Ìý Ìý 32.1 Ìý 35.4
Current liabilities Ìý Ìý Ìý (3.5) Ìý (3.9)
TOTAL POSTEMPLOYMENT BENEFIT LIABILITIES Ìý Ìý Ìý (30.3) Ìý (34.9)
Total amount recognized Ìý Ìý Ìý (1.7) Ìý (3.4)
Amounts recognized in accumulated other comprehensive income: Ìý Ìý Ìý Ìý Ìý Ìý
Prior service cost Ìý Ìý Ìý (9.9) Ìý (25.6)
Net actuarial loss Ìý Ìý Ìý 82.1 Ìý 88.3
Net amount recognized Ìý Ìý Ìý 72.2 Ìý 62.7
Salaried Employees [Member] | Pension Benefits [Member] Ìý Ìý Ìý Ìý Ìý Ìý
Defined Benefit Plan, Change in Benefit Obligation [Roll Forward] Ìý Ìý Ìý Ìý Ìý Ìý
Benefit obligations — beginning of year (368.0) Ìý Ìý Ìý Ìý Ìý
Benefit obligations — end of year (340.8) (368.0) Ìý Ìý Ìý Ìý
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward] Ìý Ìý Ìý Ìý Ìý Ìý
Fair value of plan assets — beginning of year 269.4 Ìý Ìý Ìý Ìý Ìý
Fair value of plan assets — end of year 249.8 269.4 Ìý Ìý Ìý Ìý
Funded status at December 31: Ìý Ìý Ìý Ìý Ìý Ìý
Defined Benefit Plan, Plan Assets, Amount 269.4 269.4 Ìý 249.8 Ìý 269.4
Defined Benefit Plan, Benefit Obligation (368.0) (368.0) Ìý (340.8) Ìý (368.0)
Defined Benefit Plan, Funded (Unfunded) Status of Plan Ìý Ìý Ìý (91.0) Ìý (98.6)
Salaried Employees [Member] | Other Benefits [Member] Ìý Ìý Ìý Ìý Ìý Ìý
Defined Benefit Plan, Change in Benefit Obligation [Roll Forward] Ìý Ìý Ìý Ìý Ìý Ìý
Benefit obligations — beginning of year (37.7) Ìý Ìý Ìý Ìý Ìý
Benefit obligations — end of year (32.9) (37.7) Ìý Ìý Ìý Ìý
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward] Ìý Ìý Ìý Ìý Ìý Ìý
Fair value of plan assets — beginning of year 0.0 Ìý Ìý Ìý Ìý Ìý
Fair value of plan assets — end of year 0.0 0.0 Ìý Ìý Ìý Ìý
Funded status at December 31: Ìý Ìý Ìý Ìý Ìý Ìý
Defined Benefit Plan, Plan Assets, Amount 0.0 0.0 Ìý 0.0 Ìý 0.0
Defined Benefit Plan, Benefit Obligation (37.7) (37.7) Ìý (32.9) Ìý (37.7)
Defined Benefit Plan, Funded (Unfunded) Status of Plan Ìý Ìý Ìý (32.9) Ìý (37.7)
Hourly Employees [Member] | Pension Benefits [Member] Ìý Ìý Ìý Ìý Ìý Ìý
Defined Benefit Plan, Change in Benefit Obligation [Roll Forward] Ìý Ìý Ìý Ìý Ìý Ìý
Benefit obligations — beginning of year (590.0) Ìý Ìý Ìý Ìý Ìý
Benefit obligations — end of year (548.9) (590.0) Ìý Ìý Ìý Ìý
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward] Ìý Ìý Ìý Ìý Ìý Ìý
Fair value of plan assets — beginning of year 473.0 Ìý Ìý Ìý Ìý Ìý
Fair value of plan assets — end of year 429.4 473.0 Ìý Ìý Ìý Ìý
Funded status at December 31: Ìý Ìý Ìý Ìý Ìý Ìý
Defined Benefit Plan, Plan Assets, Amount 473.0 473.0 Ìý 429.4 Ìý 473.0
Defined Benefit Plan, Benefit Obligation (590.0) (590.0) Ìý (548.9) Ìý (590.0)
Defined Benefit Plan, Funded (Unfunded) Status of Plan Ìý Ìý Ìý (119.5) Ìý (117.0)
Hourly Employees [Member] | Other Benefits [Member] Ìý Ìý Ìý Ìý Ìý Ìý
Defined Benefit Plan, Change in Benefit Obligation [Roll Forward] Ìý Ìý Ìý Ìý Ìý Ìý
Benefit obligations — beginning of year (228.2) Ìý Ìý Ìý Ìý Ìý
Benefit obligations — end of year (209.0) (228.2) Ìý Ìý Ìý Ìý
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward] Ìý Ìý Ìý Ìý Ìý Ìý
Fair value of plan assets — beginning of year 262.5 Ìý Ìý Ìý Ìý Ìý
Fair value of plan assets — end of year 240.2 262.5 Ìý Ìý Ìý Ìý
Funded status at December 31: Ìý Ìý Ìý Ìý Ìý Ìý
Defined Benefit Plan, Plan Assets, Amount 262.5 262.5 Ìý 240.2 Ìý 262.5
Defined Benefit Plan, Benefit Obligation (228.2) (228.2) Ìý (209.0) Ìý (228.2)
Defined Benefit Plan, Funded (Unfunded) Status of Plan Ìý Ìý Ìý 31.2 Ìý 34.3
Mining Employees [Member] | Pension Benefits [Member] Ìý Ìý Ìý Ìý Ìý Ìý
Defined Benefit Plan, Change in Benefit Obligation [Roll Forward] Ìý Ìý Ìý Ìý Ìý Ìý
Benefit obligations — beginning of year (10.3) Ìý Ìý Ìý Ìý Ìý
Benefit obligations — end of year (10.7) (10.3) Ìý Ìý Ìý Ìý
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward] Ìý Ìý Ìý Ìý Ìý Ìý
Fair value of plan assets — beginning of year 7.4 Ìý Ìý Ìý Ìý Ìý
Fair value of plan assets — end of year 8.0 7.4 Ìý Ìý Ìý Ìý
Funded status at December 31: Ìý Ìý Ìý Ìý Ìý Ìý
Defined Benefit Plan, Plan Assets, Amount 7.4 7.4 Ìý 8.0 Ìý 7.4
Defined Benefit Plan, Benefit Obligation (10.3) (10.3) Ìý (10.7) Ìý (10.3)
Defined Benefit Plan, Funded (Unfunded) Status of Plan Ìý Ìý Ìý (2.7) Ìý (2.9)
Supplemental Executive Retirement Plan S E R P [Member] | Pension Benefits [Member] Ìý Ìý Ìý Ìý Ìý Ìý
Defined Benefit Plan, Change in Benefit Obligation [Roll Forward] Ìý Ìý Ìý Ìý Ìý Ìý
Benefit obligations — beginning of year (4.8) Ìý Ìý Ìý Ìý Ìý
Benefit obligations — end of year (5.3) (4.8) Ìý Ìý Ìý Ìý
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward] Ìý Ìý Ìý Ìý Ìý Ìý
Fair value of plan assets — beginning of year 0.0 Ìý Ìý Ìý Ìý Ìý
Fair value of plan assets — end of year 0.0 0.0 Ìý Ìý Ìý Ìý
Funded status at December 31: Ìý Ìý Ìý Ìý Ìý Ìý
Defined Benefit Plan, Plan Assets, Amount 0.0 0.0 Ìý 0.0 Ìý 0.0
Defined Benefit Plan, Benefit Obligation $ (4.8) $ (4.8) Ìý (5.3) Ìý (4.8)
Defined Benefit Plan, Funded (Unfunded) Status of Plan Ìý Ìý Ìý $ (5.3) Ìý $ (4.8)