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

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- Definition Amount, before tax, of accumulated other comprehensive (income) loss for cost (credit) of benefit change attributable to participants' prior service from plan amendment or plan initiation of defined benefit plan, that has not been recognized in net periodic benefit cost (credit). + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 220 -SubTopic 10 -Section 45 -Paragraph 14A -URI http://asc.fasb.org/extlink&oid=114867106&loc=SL7669686-108580 Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 715 -SubTopic 20 -Section 50 -Paragraph 1 -Subparagraph (j) -URI http://asc.fasb.org/extlink&oid=108410482&loc=d3e1928-114920 + Details
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- Definition Amount of payment to participant of defined benefit plan which decreases benefit obligation. For pension plan, payment includes, but is not limited to, pension benefits and death benefits. For other postretirement plan, payment includes, but is not limited to, prescription drug benefits, health care benefits, life insurance benefits, and legal, educational and advisory services. + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 715 -SubTopic 20 -Section 50 -Paragraph 1 -Subparagraph (a)(6) -URI http://asc.fasb.org/extlink&oid=108410482&loc=d3e1928-114920 Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 715 -SubTopic 60 -Section 50 -Paragraph 4 -URI http://asc.fasb.org/extlink&oid=6414203&loc=d3e39716-114964 + Details
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