Accumulated Other Comprehensive Income (Schedule of Reclassifications) (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016
Accumulated Other Comprehensive Income (Loss) [Line Items]
Net of tax			$ (174)
Derivative instruments
Accumulated Other Comprehensive Income (Loss) [Line Items]
Net of tax			(88)
Pension, Postretirement and Other Postemployment Benefits
Accumulated Other Comprehensive Income (Loss) [Line Items]
Net of tax			(418)
Available-for-sale securities
Accumulated Other Comprehensive Income (Loss) [Line Items]
Net of tax			332
Successor
Accumulated Other Comprehensive Income (Loss) [Line Items]
Revenues	$ 613,538		1,764,379
Benefit from income taxes	1,643		484
Net income (loss)	68,351		157,179
Successor | Sale of Cumberland River pension plan
Accumulated Other Comprehensive Income (Loss) [Line Items]
Reclassification from AOCI before tax	(360)		(360)
Successor | Pension, Postretirement and Other Postemployment Benefits
Accumulated Other Comprehensive Income (Loss) [Line Items]
Reclassification from AOCI before tax	(905)		(418)
Benefit from income taxes	0		0
Net of tax	(905)		(418)
Successor | Amortization of prior service credits
Accumulated Other Comprehensive Income (Loss) [Line Items]
Reclassification from AOCI before tax	0		0
Successor | Amortization of actuarial gains (losses), net
Accumulated Other Comprehensive Income (Loss) [Line Items]
Reclassification from AOCI before tax	0		0
Successor | Pension settlement
Accumulated Other Comprehensive Income (Loss) [Line Items]
Reclassification from AOCI before tax	228		715
Successor | Actuarial curtailments
Accumulated Other Comprehensive Income (Loss) [Line Items]
Reclassification from AOCI before tax	(773)		(773)
Successor | Reclassification out of Accumulated Other Comprehensive Income | Derivative instruments
Accumulated Other Comprehensive Income (Loss) [Line Items]
Revenues	(88)		(88)
Benefit from income taxes	0		0
Net income (loss)	(88)		(88)
Successor | Reclassification out of Accumulated Other Comprehensive Income | Available-for-sale securities
Accumulated Other Comprehensive Income (Loss) [Line Items]
Interest and investment income	0		332
Benefit from income taxes	0		0
Net income (loss)	$ 0		$ 332
Predecessor
Accumulated Other Comprehensive Income (Loss) [Line Items]
Revenues		$ 550,305		$ 1,398,709
Benefit from income taxes		3,270		4,626
Net income (loss)		(51,421)		(434,010)
Predecessor | Sale of Cumberland River pension plan
Accumulated Other Comprehensive Income (Loss) [Line Items]
Reclassification from AOCI before tax				0
Predecessor | Pension, Postretirement and Other Postemployment Benefits
Accumulated Other Comprehensive Income (Loss) [Line Items]
Reclassification from AOCI before tax		(2,243)		1,844
Benefit from income taxes		0		(481)
Net of tax		(2,243)		1,363
Predecessor | Amortization of prior service credits
Accumulated Other Comprehensive Income (Loss) [Line Items]
Reclassification from AOCI before tax		2,510		7,854
Predecessor | Amortization of actuarial gains (losses), net
Accumulated Other Comprehensive Income (Loss) [Line Items]
Reclassification from AOCI before tax		(4,753)		(6,010)
Predecessor | Pension settlement
Accumulated Other Comprehensive Income (Loss) [Line Items]
Reclassification from AOCI before tax				0
Predecessor | Actuarial curtailments
Accumulated Other Comprehensive Income (Loss) [Line Items]
Reclassification from AOCI before tax				0
Predecessor | Reclassification out of Accumulated Other Comprehensive Income | Derivative instruments
Accumulated Other Comprehensive Income (Loss) [Line Items]
Revenues		76		397
Benefit from income taxes		0		(81)
Net income (loss)		76		316
Predecessor | Reclassification out of Accumulated Other Comprehensive Income | Available-for-sale securities
Accumulated Other Comprehensive Income (Loss) [Line Items]
Interest and investment income		632		(2,263)
Benefit from income taxes		0		1,038
Net income (loss)		$ 632		$ (1,225)