Condensed Consolidated Statements of Cash Flows - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2019	Mar. 31, 2018
Operating activities
Net income	$ 72,741	$ 59,985
Adjustments to reconcile to cash provided by operating activities:
Depreciation, depletion and amortization	25,273	29,703
Accretion on asset retirement obligations	5,137	6,992
Amortization of sales contracts, net	65	3,051
Deferred income taxes	0	12,127
Employee stock-based compensation expense	5,651	3,845
(Gains) losses on disposals and divestitures, net	(475)	134
Amortization relating to financing activities	907	1,080
Changes in:
Receivables	7,410	(28,728)
Inventories	(20,137)	(14,871)
Accounts payable, accrued expenses and other current liabilities	(17,861)	(26,052)
Income taxes, net	76	11,596
Other	6,197	8,005
Cash provided by operating activities	84,984	66,867
Investing activities
Capital expenditures	(39,147)	(9,453)
Minimum royalty payments	(63)	(62)
Proceeds from disposals and divestitures	608	54
Purchases of short term investments	(27,902)	(38,458)
Proceeds from sales of short term investments	26,500	49,400
Investments in and advances to affiliates, net	(2,196)	0
Cash provided by (used in) investing activities	(42,200)	1,481
Financing activities
Payments on term loan due 2024	(750)	(750)
Net payments on other debt	(4,633)	(3,431)
Dividends paid	(7,839)	(8,335)
Purchases of treasury stock	(75,749)	(38,186)
Other	0	10
Cash used in financing activities	(88,971)	(50,692)
Increase (decrease) in cash and cash equivalents, including restricted cash	(46,187)	17,656
Cash and cash equivalents, including restricted cash, beginning of period	264,937	273,602
Cash and cash equivalents, including restricted cash, end of period	218,750	291,258
Cash and cash equivalents, including restricted cash, end of period
Cash and cash equivalents, including restricted cash, end of period	$ 264,937	$ 273,602