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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, 2017 | Dec. 31, 2016 | Dec. 31, 2015 | Dec. 31, 2017 | Dec. 31, 2016
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward] |  |  |  |  |
Fair value of plan assets — beginning of year | $ 202.4 | $ 193.4 |  |  |
Employer contributions | 2.1 | 1.1 |  |  |
Fair value of plan assets — end of year | 215.6 | 202.4 | $ 193.4 |  |
Funded status at December 31: |  |  |  |  |
Defined Benefit Plan, Fair Value of Plan Assets | 202.4 | 193.4 | 193.4 | $ 215.6 | $ 202.4
Amounts recognized in Statements of Financial Position: |  |  |  |  |
Assets, Current |  |  |  | 1,544.6 | 824.6
TOTAL POSTEMPLOYMENT BENEFIT LIABILITIES |  |  |  | (257.7) | (280.5)
Pension Benefits [Member] |  |  |  |  |
Defined Benefit Plan, Change in Benefit Obligation [Roll Forward] |  |  |  |  |
Benefit obligations — beginning of year | (931.6) | (910.8) |  |  |
Service cost (excluding expenses) | 17.1 | 17.6 | 22.7 |  |
Interest cost | 30.5 | 30.3 | 37.7 |  |
Plan amendments | 0.0 | 5.7 |  |  |
Actuarial loss | (22.3) | (21.1) | (20.8) |  |
Defined Benefit Plan, Change in Actuarial (gain) loss | 54.6 | 38.1 |  |  |
Defined Benefit Plan, Benefit Obligation, Benefits Paid | 60.7 | 70.9 |  |  |
Participant contributions | 0.0 | 0.0 |  |  |
Federal subsidy on benefits paid | 0.0 | 0.0 |  |  |
Benefit obligations — end of year | (973.1) | (931.6) | (910.8) |  |
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward] |  |  |  |  |
Fair value of plan assets — beginning of year | 685.8 | 700.6 |  |  |
Actual return on plan assets | 100.2 | 54.8 |  |  |
Participant contributions | 0.0 | 0.0 |  |  |
Employer contributions | 24.4 | 1.2 |  |  |
Assets Transferred to (from) Plan | 0.1 | 0.1 |  |  |
Benefits paid | (60.7) | (70.9) |  |  |
Fair value of plan assets — end of year | 749.8 | 685.8 | 700.6 |  |
Funded status at December 31: |  |  |  |  |
Defined Benefit Plan, Fair Value of Plan Assets | 685.8 | 700.6 | 700.6 | 749.8 | 685.8
Defined Benefit Plan, Benefit Obligation | (931.6) | (910.8) | (910.8) | (973.1) | (931.6)
Defined Benefit Plan, Funded (Unfunded) Status of Plan |  |  |  | (223.3) | (245.8)
Total amount recognized |  |  |  | (223.3) | (245.8)
Amounts recognized in Statements of Financial Position: |  |  |  |  |
Assets, Current |  |  |  | 0.0 | 0.0
Current liabilities |  |  |  | (0.5) | (0.1)
TOTAL POSTEMPLOYMENT BENEFIT LIABILITIES |  |  |  | (222.8) | (245.7)
Total amount recognized |  |  |  | (223.3) | (245.8)
Amounts recognized in accumulated other comprehensive income: |  |  |  |  |
Prior service cost |  |  |  | 8.8 | 11.0
Net actuarial loss |  |  |  | 318.7 | 315.9
Net amount recognized |  |  |  | 327.5 | 326.9
The estimated amounts that will be amortized from accumulated other comprehensive income into net periodic benefit cost: |  |  |  |  |
Net actuarial loss |  |  |  | 21.1 |
Prior service cost |  |  |  | 2.2 |
Net amount recognized |  |  |  | 23.3 |
Other Benefits [Member] |  |  |  |  |
Defined Benefit Plan, Change in Benefit Obligation [Roll Forward] |  |  |  |  |
Benefit obligations — beginning of year | (264.6) | (266.0) |  |  |
Service cost (excluding expenses) | 1.8 | 1.7 | 6.4 |  |
Interest cost | 8.3 | 9.1 | 13.4 |  |
Plan amendments | 0.0 | 9.8 |  |  |
Actuarial loss | (4.5) | (6.0) | (6.6) |  |
Defined Benefit Plan, Change in Actuarial (gain) loss | 7.4 | (7.2) |  |  |
Defined Benefit Plan, Benefit Obligation, Benefits Paid | 21.4 | 21.3 |  |  |
Participant contributions | 4.6 | 6.0 |  |  |
Federal subsidy on benefits paid | 0.6 | 0.5 |  |  |
Benefit obligations — end of year | (265.9) | (264.6) | (266.0) |  |
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward] |  |  |  |  |
Fair value of plan assets — beginning of year | 253.0 | 250.6 |  |  |
Actual return on plan assets | 24.2 | 16.0 |  |  |
Participant contributions | 0.3 | 0.5 |  |  |
Employer contributions | 1.7 | 1.7 |  |  |
Assets Transferred to (from) Plan | 0.0 | 0.0 |  |  |
Benefits paid | (16.7) | (15.8) |  |  |
Fair value of plan assets — end of year | 262.5 | 253.0 | 250.6 |  |
Funded status at December 31: |  |  |  |  |
Defined Benefit Plan, Fair Value of Plan Assets | 253.0 | 250.6 | 250.6 | 262.5 | 253.0
Defined Benefit Plan, Benefit Obligation | (264.6) | (266.0) | $ (266.0) | (265.9) | (264.6)
Defined Benefit Plan, Funded (Unfunded) Status of Plan |  |  |  | (3.4) | (11.6)
Total amount recognized |  |  |  | (3.4) | (11.6)
Amounts recognized in Statements of Financial Position: |  |  |  |  |
Assets, Current |  |  |  |  | 27.3
Assets, Noncurrent |  |  |  | 35.4 |
Current liabilities |  |  |  | (3.9) | (4.1)
TOTAL POSTEMPLOYMENT BENEFIT LIABILITIES |  |  |  | (34.9) | (34.8)
Total amount recognized |  |  |  | (3.4) | (11.6)
Amounts recognized in accumulated other comprehensive income: |  |  |  |  |
Prior service cost |  |  |  | (25.6) | (26.9)
Net actuarial loss |  |  |  | 88.3 | 87.0
Net amount recognized |  |  |  | 62.7 | 60.1
The estimated amounts that will be amortized from accumulated other comprehensive income into net periodic benefit cost: |  |  |  |  |
Net actuarial loss |  |  |  | 4.9 |
Prior service cost |  |  |  | (3.0) |
Net amount recognized |  |  |  | 1.9 |
Salaried Employees [Member] | Pension Benefits [Member] |  |  |  |  |
Defined Benefit Plan, Change in Benefit Obligation [Roll Forward] |  |  |  |  |
Benefit obligations — beginning of year | (351.9) |  |  |  |
Benefit obligations — end of year | (368.0) | (351.9) |  |  |
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward] |  |  |  |  |
Fair value of plan assets — beginning of year | 242.9 |  |  |  |
Fair value of plan assets — end of year | 269.4 | 242.9 |  |  |
Funded status at December 31: |  |  |  |  |
Defined Benefit Plan, Fair Value of Plan Assets | 242.9 | 242.9 |  | 269.4 | 242.9
Defined Benefit Plan, Benefit Obligation | (351.9) | (351.9) |  | (368.0) | (351.9)
Defined Benefit Plan, Funded (Unfunded) Status of Plan |  |  |  | (98.6) | (109.0)
Salaried Employees [Member] | Other Benefits [Member] |  |  |  |  |
Defined Benefit Plan, Change in Benefit Obligation [Roll Forward] |  |  |  |  |
Benefit obligations — beginning of year | (37.6) |  |  |  |
Benefit obligations — end of year | (37.7) | (37.6) |  |  |
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, Fair Value of Plan Assets | 0.0 | 0.0 |  | 0.0 | 0.0
Defined Benefit Plan, Benefit Obligation | (37.6) | (37.6) |  | (37.7) | (37.6)
Defined Benefit Plan, Funded (Unfunded) Status of Plan |  |  |  | (37.7) | (37.6)
Hourly Employees [Member] | Pension Benefits [Member] |  |  |  |  |
Defined Benefit Plan, Change in Benefit Obligation [Roll Forward] |  |  |  |  |
Benefit obligations — beginning of year | (565.6) |  |  |  |
Benefit obligations — end of year | (590.0) | (565.6) |  |  |
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward] |  |  |  |  |
Fair value of plan assets — beginning of year | 436.9 |  |  |  |
Fair value of plan assets — end of year | 473.0 | 436.9 |  |  |
Funded status at December 31: |  |  |  |  |
Defined Benefit Plan, Fair Value of Plan Assets | 436.9 | 436.9 |  | 473.0 | 436.9
Defined Benefit Plan, Benefit Obligation | (565.6) | (565.6) |  | (590.0) | (565.6)
Defined Benefit Plan, Funded (Unfunded) Status of Plan |  |  |  | (117.0) | (128.7)
Hourly Employees [Member] | Other Benefits [Member] |  |  |  |  |
Defined Benefit Plan, Change in Benefit Obligation [Roll Forward] |  |  |  |  |
Benefit obligations — beginning of year | (227.0) |  |  |  |
Benefit obligations — end of year | (228.2) | (227.0) |  |  |
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward] |  |  |  |  |
Fair value of plan assets — beginning of year | 253.0 |  |  |  |
Fair value of plan assets — end of year | 262.5 | 253.0 |  |  |
Funded status at December 31: |  |  |  |  |
Defined Benefit Plan, Fair Value of Plan Assets | 253.0 | 253.0 |  | 262.5 | 253.0
Defined Benefit Plan, Benefit Obligation | (227.0) | (227.0) |  | (228.2) | (227.0)
Defined Benefit Plan, Funded (Unfunded) Status of Plan |  |  |  | 34.3 | 26.0
Mining Employees [Member] | Pension Benefits [Member] |  |  |  |  |
Defined Benefit Plan, Change in Benefit Obligation [Roll Forward] |  |  |  |  |
Benefit obligations — beginning of year | (10.0) |  |  |  |
Benefit obligations — end of year | (10.3) | (10.0) |  |  |
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward] |  |  |  |  |
Fair value of plan assets — beginning of year | 6.0 |  |  |  |
Fair value of plan assets — end of year | 7.4 | 6.0 |  |  |
Funded status at December 31: |  |  |  |  |
Defined Benefit Plan, Fair Value of Plan Assets | 6.0 | 6.0 |  | 7.4 | 6.0
Defined Benefit Plan, Benefit Obligation | (10.0) | (10.0) |  | (10.3) | (10.0)
Defined Benefit Plan, Funded (Unfunded) Status of Plan |  |  |  | (2.9) | (4.0)
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 | (4.1) |  |  |  |
Benefit obligations — end of year | (4.8) | (4.1) |  |  |
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, Fair Value of Plan Assets | 0.0 | 0.0 |  | 0.0 | 0.0
Defined Benefit Plan, Benefit Obligation | $ (4.1) | $ (4.1) |  | (4.8) | (4.1)
Defined Benefit Plan, Funded (Unfunded) Status of Plan |  |  |  | $ (4.8) | $ (4.1)

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- Definition Amount of contribution received by defined benefit plan from employer which increases plan assets. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 715 -SubTopic 20 -Section 50 -Paragraph 1 -Subparagraph (d)(5)(ii) -URI http://asc.fasb.org/extlink&oid=82843861&loc=d3e1928-114920 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 715 -SubTopic 20 -Section 50 -Paragraph 6 -Subparagraph (b) -URI http://asc.fasb.org/extlink&oid=82843861&loc=d3e2709-114920 Reference 3: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 715 -SubTopic 20 -Section 50 -Paragraph 1 -Subparagraph (b)(3) -URI http://asc.fasb.org/extlink&oid=82843861&loc=d3e1928-114920 Reference 4: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 715 -SubTopic 20 -Section 50 -Paragraph 1 -Subparagraph (d)(5)(iv)(01) -URI http://asc.fasb.org/extlink&oid=82843861&loc=d3e1928-114920 + Details
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- Definition Amount of cost (credit) included in accumulated other comprehensive (income) loss expected to be recognized in net periodic benefit cost (credit) for fiscal year following most recent annual statement of financial position. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 715 -SubTopic 20 -Section 50 -Paragraph 1 -Subparagraph (s) -URI http://asc.fasb.org/extlink&oid=82843861&loc=d3e1928-114920 + Details
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- Definition Amount included in accumulated other comprehensive income (loss) for gain (loss) of defined benefit plan expected to be recognized in net periodic benefit (cost) credit for fiscal year following most recent annual statement of financial position. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 715 -SubTopic 20 -Section 50 -Paragraph 1 -Subparagraph (s) -URI http://asc.fasb.org/extlink&oid=82843861&loc=d3e1928-114920 + Details
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- Definition Amount included in accumulated other comprehensive (income) loss for prior service cost (credit) expected to be recognized in net periodic benefit cost (credit) for fiscal year following most recent annual statement of financial position. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 715 -SubTopic 20 -Section 50 -Paragraph 1 -Subparagraph (s) -URI http://asc.fasb.org/extlink&oid=82843861&loc=d3e1928-114920 + Details
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- Definition Fair value of assets that have been segregated and restricted to provide pension or postretirement benefits. Assets include, but are not limited to, stocks, bonds, other investments, earnings from investments, and contributions by the employer and employees. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 715 -SubTopic 20 -Section 50 -Paragraph 1 -Subparagraph (b) -URI http://asc.fasb.org/extlink&oid=82843861&loc=d3e1928-114920 + Details
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- Definition Amount of funded (unfunded) status of defined benefit plan, measured as difference between fair value of plan assets and benefit obligation. Includes, but is not limited to, overfunded (underfunded) status. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 715 -SubTopic 20 -Section 50 -Paragraph 1 -Subparagraph (c) -URI http://asc.fasb.org/extlink&oid=82843861&loc=d3e1928-114920 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 715 -SubTopic 20 -Section 45 -Paragraph 2 -URI http://asc.fasb.org/extlink&oid=64852052&loc=d3e1703-114919 + Details
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- Definition Amount of cost recognized for passage of time related to defined benefit plan. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 715 -SubTopic 20 -Section 50 -Paragraph 1 -Subparagraph (h)(2) -URI http://asc.fasb.org/extlink&oid=82843861&loc=d3e1928-114920 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 715 -SubTopic 20 -Section 50 -Paragraph 1 -Subparagraph (a)(2) -URI http://asc.fasb.org/extlink&oid=82843861&loc=d3e1928-114920 + Details
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- Definition Amount of increase (decrease) in benefit obligation of defined benefit plan from change in terms of existing plan or initiation of new plan. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 715 -SubTopic 20 -Section 50 -Paragraph 1 -Subparagraph (a)(7) -URI http://asc.fasb.org/extlink&oid=82843861&loc=d3e1928-114920 + Details
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- Definition Amount of payment to participant under defined benefit plan which decreases plan assets. 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://www.xbrl.org/2003/role/presentationRef -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 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 715 -SubTopic 20 -Section 50 -Paragraph 1 -Subparagraph (b)(5) -URI http://asc.fasb.org/extlink&oid=82843861&loc=d3e1928-114920 + Details
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