Coal Trading - Trading Revenues and Offsetting (Details) - Coal Trading - USD ($)	3 Months Ended			6 Months Ended
	Jun. 30, 2017	Apr. 01, 2017	Jun. 30, 2016	Jun. 30, 2016	Dec. 31, 2016
Coal trading derivative instruments and balance sheet offsetting disclosures:
Net variation margin (held) posted	$ 16,000,000				$ 57,400,000
Cash Flow Hedging
Coal trading derivative instruments and balance sheet offsetting disclosures:
Net variation margin (held) posted	0				0
Successor
Trading Revenues by Type of Instrument [Line Items]
Total trading revenues (losses)	5,200,000
Coal trading derivative instruments and balance sheet offsetting disclosures:
Assets from coal trading activities, gross amounts of recognized assets	145,500,000
Liabilities from coal trading activities, gross amounts of recognized liabilities	(162,100,000)
Assets and (liabilities) from coal trading activities, net amounts recognized before the application of variation margin	(16,600,000)
Gross amounts of coal trading liabilities offset against associated coal trading assets	(144,900,000)
Gross amounts of coal trading assets offset against associated coal trading liabilities	144,900,000
Net coal trading (liabilities) assets offset against associated assets (liabilities)	0
Variation margin held offset against assets from coal trading activities	0
Variation margin posted offset against liabilities from coal trading activities	16,000,000
Net variation margin (held) posted	16,000,000
Assets from coal trading activities, net	600,000
Liabilities from coal trading activities, net	(1,200,000)
Net assets (liabilities) from coal trading activities	(600,000)
Predecessor
Trading Revenues by Type of Instrument [Line Items]
Total trading revenues (losses)		$ 15,000,000	$ 17,500,000	$ 13,800,000
Coal trading derivative instruments and balance sheet offsetting disclosures:
Assets from coal trading activities, gross amounts of recognized assets					191,200,000
Liabilities from coal trading activities, gross amounts of recognized liabilities					(249,100,000)
Assets and (liabilities) from coal trading activities, net amounts recognized before the application of variation margin					(57,900,000)
Gross amounts of coal trading liabilities offset against associated coal trading assets					(190,500,000)
Gross amounts of coal trading assets offset against associated coal trading liabilities					190,500,000
Net coal trading (liabilities) assets offset against associated assets (liabilities)					0
Variation margin held offset against assets from coal trading activities					0
Variation margin posted offset against liabilities from coal trading activities					57,400,000
Net variation margin (held) posted					57,400,000
Assets from coal trading activities, net					700,000
Liabilities from coal trading activities, net					(1,200,000)
Net assets (liabilities) from coal trading activities					$ (500,000)
Futures, swaps and options | Successor
Trading Revenues by Type of Instrument [Line Items]
Total trading revenues (losses)	(7,300,000)
Futures, swaps and options | Predecessor
Trading Revenues by Type of Instrument [Line Items]
Total trading revenues (losses)		(10,200,000)	(19,300,000)	(23,100,000)
Physical purchase/sale contracts | Successor
Trading Revenues by Type of Instrument [Line Items]
Total trading revenues (losses)	$ 12,500,000
Physical purchase/sale contracts | Predecessor
Trading Revenues by Type of Instrument [Line Items]
Total trading revenues (losses)		$ 25,200,000	$ 36,800,000	$ 36,900,000