Segment Information - Reconciliation of Net Income (Loss) (Details) - USD ($) $ in Thousands	3 Months Ended			9 Months Ended
	Sep. 30, 2020	Sep. 30, 2019	Mar. 31, 2019	Sep. 30, 2020	Sep. 30, 2019
Revenue from External Customer [Line Items]
Segment Adjusted EBITDA	$ 23,859	$ 90,620		$ 121,949	$ 459,912
Other revenues	180,064	287,506		570,484	1,063,408
Cost of other revenues	(7,064)	(7,583)		(22,267)	(23,346)
Depreciation and depletion	(27,965)	(25,741)		(78,813)	(73,652)
Selling, general and administrative	(8,192)	(9,362)		(25,105)	(29,050)
Loss on early extinguishment of debt	0	0	$ (9,800)	0	(9,756)
Other income	0	5,272		1,822	22,815
Interest expense, net	(8,059)	(7,250)		(23,847)	(22,793)
Income tax benefit (expense)	8,152	(7,599)		9,336	(68,639)
Net income (loss)	(14,434)	45,022		(2,050)	280,949
Other revenues
Revenue from External Customer [Line Items]
Other revenues	$ 4,835	$ 6,665		$ 14,875	$ 25,458