Condensed Consolidated Statements of Cash Flows - USD ($) $ in Thousands	6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017
Operating activities
Net income	$ 103,291	$ 88,828
Adjustments to reconcile to cash provided by operating activities:
Depreciation, depletion and amortization	60,252	62,622
Accretion on asset retirement obligations	13,985	15,246
Amortization of sales contracts, net	6,299	29,042
Prepaid royalties expensed	0	2,288
Deferred income taxes	8,730	5,996
Employee stock-based compensation expense	7,992	4,942
Gains on disposals and divestitures, net	131	(2,005)
Net loss resulting from early retirement of debt and debt restructuring	485	2,061
Amortization relating to financing activities	2,170	1,565
Changes in:
Receivables	(20,212)	(3,864)
Inventories	(28,245)	(23,594)
Accounts payable, accrued expenses and other current liabilities	(11,879)	(89)
Income taxes, net	11,560	(3,796)
Other	(9,563)	21,557
Cash provided by operating activities	144,996	200,799
Investing activities
Capital expenditures	(30,049)	(16,922)
Minimum royalty payments	(124)	(4,211)
Proceeds from disposals and divestitures	56	4,186
Purchases of short term investments	(110,359)	(157,364)
Proceeds from sales of short term investments	105,150	85,035
Investments in and advances to affiliates, net	0	(8,934)
Cash used in investing activities	(35,326)	(98,210)
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	(1,500)	(750)
Net payments on other debt	(7,307)	(5,207)
Debt financing costs	(529)	(8,900)
Net loss resulting from early retirement of debt and debt restructuring	(50)	(2,061)
Dividends paid	(16,333)	(8,563)
Purchases of treasury stock	(115,973)	(51,043)
Other	10
Cash used in financing activities	(141,682)	(103,708)
Decrease in cash and cash equivalents, including restricted cash	(32,012)	(1,119)
Cash and cash equivalents, including restricted cash, beginning of period	273,602	376,422
Cash and cash equivalents, including restricted cash, end of period	241,590	375,303
Cash and cash equivalents, including restricted cash, end of period
Cash and cash equivalents, including restricted cash	$ 273,602	$ 376,422