Consolidated Statements of Cash Flows - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Operating activities
Net income	$ 233,799	$ 312,577	$ 238,450
Adjustments to reconcile net income to cash provided by (used in) operating activities:
Depreciation, depletion and amortization	112,055	119,563	122,464
Accretion on asset retirement obligations	20,548	27,970	30,209
Amortization of sales contracts, net	(434)	11,107	53,985
Prepaid royalties expensed	4,270	134	2,905
Deferred income taxes	13,501	18,701	(21,965)
Employee stock-based compensation expense	21,989	17,519	10,437
Loss (Gain) on disposals and divestitures	8,304	(625)	(24,327)
Amortization relating to financing activities	3,691	4,179	3,736
Preference Rights Lease Application settlement income	(39,000)	0	0
Net loss resulting from early retirement of debt and debt restructuring	0	485	2,547
Changes in:
Receivables	30,713	(22,903)	8,370
Inventories	(15,251)	3,490	(19,626)
Income taxes receivable/payable	38,152	(46,971)	(6,834)
Coal derivative assets and liabilities, including margin account	10,117	(779)	(7,342)
Accounts payable, accrued expenses and other current liabilities	(28,222)	(14,208)	17,173
Asset retirement obligations	(11,455)	(9,743)	(20,584)
Pension, postretirement and other postemployment benefits	(209)	(4,703)	(15,253)
Other	17,146	2,170	22,129
Cash provided by operating activities	419,714	417,963	396,474
Investing activities
Capital expenditures	(266,356)	(95,272)	(59,205)
Minimum royalty payments	(1,249)	(584)	(5,296)
Proceeds from disposals and divestitures	6,135	1,083	12,920
Purchases of short term investments	(205,216)	(143,328)	(258,948)
Proceeds from sales of short term investments	233,074	136,630	190,064
Investments in and advances to affiliates, net	(5,499)	(2,481)	(10,173)
Cash used in investing activities	(239,111)	(103,952)	(130,638)
Financing activities
Proceeds from issuance of term loan due 2024	0	0	298,500
Payments to extinguish term loan due 2021	0	0	(325,684)
Payments on term loan	(3,000)	(3,000)	(2,250)
Net payments on other debt	(5,373)	(6,077)	(694)
Debt financing costs	0	(1,257)	(10,149)
Dividends paid	(30,220)	(31,269)	(24,369)
Purchases of treasury stock	(244,998)	(280,871)	(301,512)
Payments for taxes related to net share settlement of equity awards	(8,961)	0	0
Expenses related to debt restructuring	0	(50)	(2,360)
Other	32	(152)	(138)
Cash provided by (used in) financing activities	(292,520)	(322,676)	(368,656)
Decrease in cash and cash equivalents, including restricted cash	(111,917)	(8,665)	(102,820)
Cash and cash equivalents, including restricted cash, beginning of period	264,937	273,602	376,422
Cash and cash equivalents, including restricted cash, end of period	153,020	264,937	273,602
SUPPLEMENTAL CASH FLOW INFORMATION
Cash paid during the period for interest	16,627	17,493	34,691
Cash refunded during the period for income taxes, net	$ 52,272	$ 24,330	$ 7,958