CONSOLIDATED STATEMENTS OF CASH FLOWS (Unaudited) - USD ($) $ in Thousands	6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017
OPERATING ACTIVITIES
Net income	$ 112,073	$ 47,474
Adjustments to reconcile net income to net cash provided by operating activities:
Depreciation, amortization and other non-cash charges	79,410	87,379
Stock-based compensation expense	4,586	6,012
Gain on disposition of assets, net	(3,173)	(5,276)
Deferred income tax provision (benefit)	5,171	(20,759)
(Increase) decrease in assets:
Accounts receivable	843	(174,220)
Income taxes receivable	18,984	48,248
Inventories	(11,880)	(4,193)
Prepaid expenses	2,607	1,309
Other current assets	990	310
Other non-current assets	(1,113)	(2,256)
Increase (decrease) in liabilities:
Accounts payable	22,892	27,760
Income taxes payable	4,342	10,277
Accrued payroll and related expenses	515	4,742
Accrued insurance expenses	293	252
Accrued state, local and other taxes	(2,751)	3,547
Other accrued expenses	160	(5,024)
Pension liabilities	(4,367)	2,479
Long-term accrued insurance expenses	440	389
Other long-term liabilities	(818)	219
Net cash provided by operating activities	229,204	28,669
INVESTING ACTIVITIES
Capital expenditures	(149,692)	(30,645)
Proceeds from sale of assets	6,895	8,407
Net cash used for investing activities	(142,797)	(22,238)
FINANCING ACTIVITIES
Payment of dividends	(43,186)
Cash paid for common stock purchased and retired	(40,096)	(12,454)
Net cash used for financing activities	(83,282)	(12,454)
Net increase (decrease) in cash and cash equivalents	3,125	(6,023)
Cash and cash equivalents at beginning of period	91,050	131,835
Cash and cash equivalents at end of period	94,175	125,812
Supplemental cash flows disclosure:
Interest paid, net of amounts capitalized	133	72
Income taxes paid (refund), net	4,977	(14,119)
Supplemental disclosure of noncash investing activities:
Capital expenditures included in accounts payable	$ 19,727	$ 8,866