Consolidated Statements of Cash Flows - USD ($) $ in Thousands	6 Months Ended
	Jun. 30, 2016	Jun. 30, 2015
OPERATING ACTIVITIES
Net income	$ 30,745	$ 40,928
Adjustments to reconcile net income to net cash provided by operating activities:
Depreciation and amortization	51,552	53,375
Deferred income taxes	(5,341)	(2,063)
Amortization of stock-based compensation, net of tax	(664)	(714)
Gain on disposal of property and equipment	(5,800)	(19,849)
Changes in certain working capital items:
Trade receivables	4,000	6,574
Prepaid expenses and other current assets	308	1,368
Accounts payable, accrued liabilities, and accrued expenses	5,696	6,441
Accrued income taxes	(3,927)	14,222
Net cash provided by operating activities	77,897	101,710
INVESTING ACTIVITIES
Proceeds from sale of property and equipment	26,744	57,349
Purchases of property and equipment, net of trades	(42,353)	(64,703)
Change in designated funds for equipment purchases	0	(5,741)
Change in other assets	97	1,682
Net cash used in investing activities	(15,512)	(11,413)
FINANCING ACTIVITIES
Payments of cash dividends	(3,332)	(1,759)
Contingent consideration related to acquisition	0	(1,765)
Repayments on line of credit	0	(24,600)
Repurchases of common stock	(14,678)	0
Net cash used in financing activities	(18,010)	(28,124)
Net increase in cash and cash equivalents	44,375	62,173
CASH AND CASH EQUIVALENTS
Beginning of period	33,232
End of period	77,607	79,476
SUPPLEMENTAL DISCLOSURES OF CASH FLOW INFORMATION
Interest paid	0	40
Cash paid during the period for income taxes, net of refunds	23,384	10,802
Noncash investing and financing activities:
Purchased property and equipment in accounts payable	9,334	5,682
Sold property and equipment in other current assets	2,501	1,598
Common stock dividends declared in accounts payable	$ 0	$ 1,760