Accumulated Other Comprehensive Income (Loss) (Schedule of Reclassifications) (Details) - USD ($) $ in Thousands		3 Months Ended								9 Months Ended	12 Months Ended
	Oct. 01, 2016	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Oct. 01, 2016	Dec. 31, 2017	Dec. 31, 2015
Accumulated Other Comprehensive Income (Loss) [Line Items]
Revenues						$ 575,688					$ 2,324,623
Interest expense						(10,754)					(24,256)
Provision for (benefit from) income taxes						(1,156)					35,255
Income (loss) before income taxes						34,605					203,195
Net income (loss)						33,449					238,450
Successor
Accumulated Other Comprehensive Income (Loss) [Line Items]
Revenues		$ 560,244				575,688					2,324,623
Revenues, coal hedges						174,097					793,594
Interest expense						(10,754)					(24,256)
Provision for (benefit from) income taxes						(1,156)					35,255
Income (loss) before income taxes						34,605					203,195
Net income (loss)		$ 81,271				33,449
Successor | Pension, Postretirement and Other Post-Employment Benefits
Accumulated Other Comprehensive Income (Loss) [Line Items]
Provision for (benefit from) income taxes						0					0
Amortization of prior service credits						0					0
Amortization of net actuarial gains (losses)						0					0
Curtailments						0					(773)
Settlements						0					1,531
Income (loss) before income taxes						0					758
Net income (loss)						0					758
Successor | Reclassification out of accumulated other comprehensive income | Derivative Instruments | Commodity Contract
Accumulated Other Comprehensive Income (Loss) [Line Items]
Revenues
Revenues, coal hedges						0					0
Interest expense											(150)
Provision for (benefit from) income taxes						0					0
Net income (loss)						0					(150)
Successor | Reclassification out of accumulated other comprehensive income | Available-for-Sale Securities
Accumulated Other Comprehensive Income (Loss) [Line Items]
Provision for (benefit from) income taxes						0					0
Interest and investment income						0					387
Net income (loss)						$ 0					$ 387
Predecessor
Accumulated Other Comprehensive Income (Loss) [Line Items]
Revenues			$ 613,538	$ 549,866	$ 600,975		$ 550,305	$ 420,298	$ 428,106	$ 1,398,709		$ 2,573,260
Revenues, coal hedges										280,879		395,323
Interest expense										(133,235)		(393,549)
Provision for (benefit from) income taxes										4,626		373,380
Income (loss) before income taxes										1,237,455		(3,286,522)
Net income (loss)	$ 1,676,091		$ 68,351	$ 37,160	$ 51,668		$ (51,421)	$ (175,887)	$ (206,702)	1,242,081		$ (2,913,142)
Predecessor | Pension, Postretirement and Other Post-Employment Benefits
Accumulated Other Comprehensive Income (Loss) [Line Items]
Provision for (benefit from) income taxes										(481)
Amortization of prior service credits										7,854
Amortization of net actuarial gains (losses)										(6,010)
Curtailments										0
Settlements										0
Income (loss) before income taxes										1,844
Net income (loss)										1,363
Predecessor | Reclassification out of accumulated other comprehensive income | Derivative Instruments | Commodity Contract
Accumulated Other Comprehensive Income (Loss) [Line Items]
Revenues
Revenues, coal hedges										397
Interest expense										0
Provision for (benefit from) income taxes										(81)
Net income (loss)										316
Predecessor | Reclassification out of accumulated other comprehensive income | Available-for-Sale Securities
Accumulated Other Comprehensive Income (Loss) [Line Items]
Provision for (benefit from) income taxes										1,038
Interest and investment income										(2,263)
Net income (loss)										$ (1,225)