Taxes (Schedule of Effective Income Tax Rate Reconciliation) (Details) - USD ($) $ in Thousands	3 Months Ended	9 Months Ended	12 Months Ended
	Dec. 31, 2016	Oct. 01, 2016	Dec. 31, 2018	Dec. 31, 2017
Income Tax Examination [Line Items]
Income tax provision (benefit) at statutory rate	$ 12,112		$ 54,621	$ 71,118
Percentage depletion allowance	(4,292)		(17,725)	(31,255)
State taxes, net of effect of federal taxes	633		4,480	7,002
Reversal of cancellation of indebtedness income	0		0	0
Worthless stock deduction	0		0	0
Change in valuation allowance	(7,655)		(79,961)	(410,983)
Impact of Tax Cuts and Jobs Act of 2017	0		(17,645)	332,345
Other, net	358		3,754	(3,482)
Provision for (benefit from) income taxes	$ 1,156		$ (52,476)	$ (35,255)
Predecessor
Income Tax Examination [Line Items]
Income tax provision (benefit) at statutory rate		$ 433,109
Percentage depletion allowance		(3,681)
State taxes, net of effect of federal taxes		(46,122)
Reversal of cancellation of indebtedness income		(1,493,162)
Worthless stock deduction		(80,077)
Change in valuation allowance		1,185,326
Impact of Tax Cuts and Jobs Act of 2017		0
Other, net		(19)
Provision for (benefit from) income taxes		$ (4,626)