Condensed Consolidated Statements of Cash Flows - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Operating activities
Net income (loss)	$ (344,615)	$ 233,799	$ 312,577
Adjustments to reconcile to cash from operating activities:
Depreciation, depletion and amortization	121,552	111,621	130,670
Accretion on asset retirement obligations	19,887	20,548	27,970
Deferred income taxes	14,430	13,501	18,701
Employee stock-based compensation expense	17,435	21,989	17,519
Amortization relating to financing activities	5,599	3,691	4,179
Gain on property insurance recovery related to Mountain Laurel longwall	(23,518)	0
Loss (Gain) on disposals and divestitures, net	(3,727)	8,304	(625)
Non-cash asset impairment and restructuring	198,007	0
Preference Rights Lease Application settlement income	0	(39,000)
Changes in:
Receivables	63,657	30,713	(22,903)
Inventories	(9,126)	(15,251)	3,490
Income taxes, net	22,859	38,152	(46,971)
Coal derivative assets and liabilities, including margin account	(1,045)	10,117	(779)
Accounts payable, accrued expenses and other current liabilities	(46,066)	(28,222)	(14,208)
Asset retirement obligations	(16,144)	(11,455)	(9,743)
Pension, postretirement and other postemployment benefits	588	(209)	(4,703)
Other	41,333	21,416	2,789
Cash provided by operating activities	61,106	419,714	417,963
Investing activities
Capital expenditures	(285,821)	(266,356)	(95,272)
Minimum royalty payments	(1,248)	(1,249)	(584)
Proceeds from disposals and divestitures	1,007	6,135	1,083
Purchases of short term investments	(120,624)	(205,216)	(143,328)
Proceeds from sales of short term investments	158,708	233,074	136,630
Investments in and advances to affiliates, net	(1,549)	(5,499)	(2,481)
Proceeds from property insurance recovery related to Mountain Laurel longwall	23,518	0
Cash used in investing activities	(226,009)	(239,111)	(103,952)
Financing activities
Payments on term loan	(3,000)	(3,000)	(3,000)
Proceeds from equipment financing	53,611	0
Proceeds from tax exempt bonds	53,090	0
Proceeds from convertible debt	155,250	0
Purchase of capped call related to convertible debt	(17,543)	0
Net payments on other debt	(15,922)	(5,373)	(6,077)
Debt financing costs	(9,718)	0	(1,257)
Dividends paid	(8,245)	(30,220)	(31,269)
Purchases of treasury stock	0	(244,998)	(280,871)
Payments for taxes related to net share settlement of equity awards	(2,195)	(8,961)
Other	0	32	(202)
Cash provided by (used in) financing activities	205,328	(292,520)	(322,676)
Increase (decrease) in cash and cash equivalents, including restricted cash	40,425	(111,917)	(8,665)
Cash and cash equivalents, including restricted cash, beginning of period	153,020	264,937	273,602
Cash and cash equivalents, including restricted cash, end of period	193,445	153,020	264,937
SUPPLEMENTAL CASH FLOW INFORMATION
Cash paid during the period for interest	19,602	16,627	17,493
Restricted cash	5,953	0	215
Cash refunded during the period for income taxes, net	$ 37,535	$ 52,272	$ 24,330