Condensed Consolidated Statements of Cash Flows - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017
Operating activities
Net income	$ 59,985	$ 51,668
Adjustments to reconcile to cash provided by operating activities:
Depreciation, depletion and amortization	29,703	31,921
Accretion on asset retirement obligations	6,992	7,623
Amortization of sales contracts, net	3,051	14,690
Prepaid royalties expensed	0	2,281
Deferred income taxes	12,127	5,830
Employee stock-based compensation expense	3,845	2,426
Gains on disposals and divestitures, net	134	(347)
Net loss resulting from early retirement of debt and debt restructuring	0	2,030
Amortization relating to financing activities	1,080	535
Changes in:
Receivables	(28,728)	37,134
Inventories	(14,871)	(11,732)
Accounts payable, accrued expenses and other current liabilities	(26,052)	(20,529)
Income taxes, net	11,596	(4,965)
Other	8,005	6,964
Cash provided by operating activities	66,867	125,529
Investing activities
Capital expenditures	(9,453)	(5,950)
Minimum royalty payments	(62)	(63)
Proceeds from disposals and divestitures	54	420
Purchases of short term investments	(38,458)	(78,523)
Proceeds from sales of short term investments	49,400	45,886
Investments in and advances to affiliates, net	0	(7,905)
Cash provided by (used in) investing activities	1,481	(46,135)
Financing activities
Proceeds from issuance of term loan due 2024	0	298,500
Payments to extinguish term loan due 2021	0	(325,684)
Payments on term loan due 2024	(750)	0
Net payments on other debt	(3,431)	(2,810)
Debt financing costs	0	(7,228)
Net loss resulting from early retirement of debt and debt restructuring	0	(2,030)
Dividends paid	(8,335)	0
Purchases of treasury stock	(38,186)	0
Other	10
Cash used in financing activities	(50,692)	(39,252)
Increase in cash and cash equivalents, including restricted cash	17,656	40,142
Cash and cash equivalents, including restricted cash, beginning of period	273,602	376,422
Cash and cash equivalents, including restricted cash, end of period	$ 291,258	$ 416,564