Consolidated Statements Of Cash Flows - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017
Cash flows from operating activities:
Net income	$ 27,807	$ 16,019
Adjustments to reconcile net income to net cash provided by operating activities:
Depreciation	55,506	55,336
Deferred income taxes	5,478	3,433
Gain on disposal of property and equipment	(2,708)	(1,392)
Non-cash equity compensation	1,410	896
Insurance and claims accruals, net of current portion	290	(2,915)
Other	(1,779)	(3,897)
Changes in certain working capital items:
Accounts receivable, net	2,207	14,337
Other current assets	5,320	11,269
Accounts payable	3,822	(1,869)
Other current liabilities	2,509	(5,289)
Net cash provided by operating activities	99,862	85,928
Cash flows from investing activities:
Additions to property and equipment	(94,237)	(42,659)
Proceeds from sales of property and equipment	38,731	28,065
Decrease in notes receivable	5,944	7,206
Net cash used in investing activities	(49,562)	(7,388)
Cash flows from financing activities:
Repayments of short-term debt	0	(20,000)
Repayments of long-term debt	0	(30,000)
Change in net checks issued in excess of cash balances	(21,539)	0
Dividends on common stock	(5,068)	(4,330)
Tax withholding related to net share settlements of restricted stock awards	(667)	(341)
Stock options exercised	229	524
Net cash used in financing activities	(27,045)	(54,147)
Effect of exchange rate fluctuations on cash	403	430
Net increase in cash, cash equivalents and restricted cash	23,658	24,823
Cash, cash equivalents and restricted cash, beginning of period	15,131	17,477
Cash, cash equivalents and restricted cash, end of period	38,789	42,300
Supplemental disclosures of cash flow information:
Interest paid	481	876
Income taxes paid	301	378
Supplemental schedule of non-cash investing activities:
Notes receivable issued upon sale of property and equipment	2,178	872
Change in fair value of interest rate swap	278	214
Property and equipment acquired included in accounts payable	5,999	671
Property and equipment disposed included in other receivables	$ 67	$ 11