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Coal Trading (Tables) - Coal Trading [Member] | 12 Months Ended
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Dec. 31, 2017
Coal Trading [Line Items] |
Trading revenue by type of instrument | Trading revenues recognized during the periods presented below were as follows:
|  | Successor | Predecessor
Trading Revenues by Type of Instrument |  | April 2 through December 31, 2017 | January 1 through April 1, 2017 |  | Year Ended December 31, 2016 |  | Year Ended December 31, 2015
|  | (Dollars in millions)
Futures, swaps and options |  | $ | (37.7 | ) | $ | (10.2 | ) |  | $ | (66.5 | ) |  | $ | 107.3 |
Physical purchase/sale contracts |  | 71.3 |  | 25.2 |  |  | 95.4 |  |  | (66.7 | )
Total trading revenues |  | $ | 33.6 |  | $ | 15.0 |  |  | $ | 28.9 |  |  | $ | 40.6 |
Schedule of Derivative Instruments [Table Text Block] | The fair value of assets and liabilities from coal trading activities presented on a gross and net basis as of December 31, 2017 and 2016 is set forth below:
Affected line item in the consolidated balance sheets |  | Gross Amounts of Recognized Assets (Liabilities) |  | Gross Amounts Offset in the Consolidated Balance Sheets |  | Variation margin posted (1) |  | Net Amounts of Assets (Liabilities) Presented in the Consolidated Balance Sheets
|  | (Dollars in millions)
|  | Successor
|  | Fair Value as of December 31, 2017
Assets from coal trading activities, net |  | $ | 77.1 |  |  | $ | (74.5 | ) |  | $ | — |  |  | $ | 2.6 |
Liabilities from coal trading activities, net |  | (122.0 | ) |  | 74.5 |  |  | 35.8 |  |  | (11.7 | )
Total, net |  | $ | (44.9 | ) |  | $ | — |  |  | $ | 35.8 |  |  | $ | (9.1 | )
|  | Predecessor
|  | Fair Value as of December 31, 2016
Assets from coal trading activities, net |  | $ | 191.2 |  |  | $ | (190.5 | ) |  | $ | — |  |  | $ | 0.7 |
Liabilities from coal trading activities, net |  | (249.1 | ) |  | 190.5 |  |  | 57.4 |  |  | (1.2 | )
Total, net |  | $ | (57.9 | ) |  | $ | — |  |  | $ | 57.4 |  |  | $ | (0.5 | )
(1) | None of the net variation margin posted at December 31, 2017 and 2016, respectively, related to cash flow hedges.
Fair value coal trading net assets (liabilities) measured on recurring basis | The following tables set forth the hierarchy of the Company’s net financial asset (liability) coal trading positions for which fair value is measured on a recurring basis as of December 31, 2017 and 2016:
| Successor
| December 31, 2017
| Level 1 |  | Level 2 |  | Level 3 |  | Total
| (Dollars in millions)
Futures, swaps and options | $ | (3.0 | ) |  | $ | (4.2 | ) |  | $ | — |  |  | $ | (7.2 | )
Physical purchase/sale contracts | — |  |  | (1.9 | ) |  | — |  |  | (1.9 | )
Total net financial liabilities | $ | (3.0 | ) |  | $ | (6.1 | ) |  | $ | — |  |  | $ | (9.1 | )
| Predecessor
| December 31, 2016
| Level 1 |  | Level 2 |  | Level 3 |  | Total
| (Dollars in millions)
Futures, swaps and options | $ | — |  |  | $ | (0.1 | ) |  | $ | — |  |  | $ | (0.1 | )
Physical purchase/sale contracts | — |  |  | 0.7 |  |  | (1.1 | ) |  | (0.4 | )
Total net financial assets (liabilities) | $ | — |  |  | $ | 0.6 |  |  | $ | (1.1 | ) |  | $ | (0.5 | )
Change in the Company's recurring Level 3 net financial assets | The following table summarizes the changes in the Company’s recurring Level 3 net financial liabilities:
| Successor | Predecessor
| April 2 through December 31, 2017 | January 1 through April 1, 2017 |  | Year Ended December 31, 2016 |  | Year Ended December 31, 2015
| (Dollars in millions)
Beginning of period | $ | (0.7 | ) | $ | (1.1 | ) |  | $ | (15.6 | ) |  | $ | 2.1 |
Transfers into Level 3 | — |  | — |  |  | 5.3 |  |  | (4.4 | )
Transfers out of Level 3 | 0.7 |  | 0.2 |  |  | (0.4 | ) |  | — |
Total gains realized/unrealized: |  |  |  |  |  |
Included in earnings | — |  | 0.2 |  |  | (2.4 | ) |  | (10.1 | )
Purchases | — |  | — |  |  | — |  |  | (0.5 | )
Sales | — |  | — |  |  | — |  |  | (0.1 | )
Settlements | — |  | — |  |  | 12.0 |  |  | (2.6 | )
End of period | $ | — |  | $ | (0.7 | ) |  | $ | (1.1 | ) |  | $ | (15.6 | )
Changes in unrealized gains (losses) relating to Level 3 net financial assets | The following table summarizes the changes in net unrealized gains (losses) relating to Level 3 net financial liabilities held both as of the beginning and the end of the period:
| Successor | Predecessor
| April 2 through December 31, 2017 | January 1 through April 1, 2017 |  | Year Ended December 31, 2016 |  | Year Ended December 31, 2015
| (Dollars in millions)
Changes in unrealized gains (losses) (1) | $ | — |  | $ | 0.3 |  |  | $ | — |  |  | $ | (6.2 | )
(1) | Within the consolidated statements of operations and consolidated statements of comprehensive income for the periods presented, unrealized gains and losses from Level 3 items are combined with unrealized gains and losses on positions classified in Level 1 or 2, as well as other positions that have been realized during the applicable periods.

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- Definition Tabular disclosure of the amount of gains or losses for the period included in earnings or resulted in a change to net assets, have arisen from the use of significant unobservable inputs (level 3) to measure the fair value of assets, liabilities, and financial instruments classified in shareholders' equity. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 820 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (d) -URI http://asc.fasb.org/extlink&oid=66048111&loc=d3e19207-110258 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 820 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (c)(1) -URI http://asc.fasb.org/extlink&oid=66048111&loc=d3e19207-110258 + Details
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- Definition Tabular disclosure of the fair value measurement of assets using significant unobservable inputs (Level 3), a reconciliation of the beginning and ending balances, separately presenting changes during the period attributable to the following: (1) total gains or losses for the period (realized and unrealized), segregating those gains or losses included in earnings (or changes in net assets) and gains or losses recognized in other comprehensive income (loss), and a description of where those gains or losses included in earnings (or changes in net assets) are reported in the statement of income (or activities); (2) purchases, sales, issues, and settlements (each type disclosed separately); and (3) transfers in and transfers out of Level 3 (for example, transfers due to changes in the observability of significant inputs), by class of asset. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 820 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (c) -URI http://asc.fasb.org/extlink&oid=66048111&loc=d3e19207-110258 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 820 -SubTopic 10 -Section 50 -Paragraph 3 -URI http://asc.fasb.org/extlink&oid=66048111&loc=d3e19279-110258 + Details
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- Definition Tabular disclosure of pertinent information about a derivative or group of derivatives on a disaggregated basis, such as for individual instruments, or small groups of similar instruments. May include a combination of the type of instrument, risks being hedged, notional amount, hedge designation, related hedged item, inception date, maturity date, or other relevant item. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 815 -SubTopic 10 -Section 50 -Paragraph 5 -URI http://asc.fasb.org/extlink&oid=84234895&loc=d3e41641-113959 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 815 -SubTopic 10 -Section 50 -Paragraph 4B -URI http://asc.fasb.org/extlink&oid=84234895&loc=SL5624163-113959 Reference 3: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 815 -SubTopic 10 -Section 50 -Paragraph 8 -URI http://asc.fasb.org/extlink&oid=84234895&loc=d3e41678-113959 Reference 4: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 815 -SubTopic 10 -Section 50 -Paragraph 4 -URI http://asc.fasb.org/extlink&oid=84234895&loc=d3e41638-113959 Reference 5: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 815 -SubTopic 10 -Section 50 -Paragraph 1B -URI http://asc.fasb.org/extlink&oid=84234895&loc=SL5580258-113959 Reference 6: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 815 -SubTopic 10 -Section 50 -Paragraph 1A -URI http://asc.fasb.org/extlink&oid=84234895&loc=SL5579245-113959 Reference 7: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 815 -SubTopic 10 -Section 50 -Paragraph 4C -URI http://asc.fasb.org/extlink&oid=84234895&loc=SL5624171-113959 Reference 8: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 815 -SubTopic 10 -Section 50 -Paragraph 1 -URI http://asc.fasb.org/extlink&oid=84234895&loc=SL5579240-113959 Reference 9: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 815 -SubTopic 10 -Section 50 -Paragraph 2 -URI http://asc.fasb.org/extlink&oid=84234895&loc=d3e41620-113959 Reference 10: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 235 -SubTopic 10 -Section S99 -Paragraph 1 -Subparagraph (SX 210.4-08.(n)(2)) -URI http://asc.fasb.org/extlink&oid=26873400&loc=d3e23780-122690 + Details
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- Definition Tabular disclosure of assets and liabilities, including [financial] instruments measured at fair value that are classified in stockholders' equity, if any, that are measured at fair value on a recurring basis. The disclosures contemplated herein include the fair value measurements at the reporting date by the level within the fair value hierarchy in which the fair value measurements in their entirety fall, segregating fair value measurements using quoted prices in active markets for identical assets (Level 1), significant other observable inputs (Level 2), and significant unobservable inputs (Level 3). + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 820 -SubTopic 10 -Section 50 -Paragraph 1 -URI http://asc.fasb.org/extlink&oid=66048111&loc=d3e19190-110258 + Details
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