Consolidated Statements Of Cash Flows - USD ($) $ in Thousands	6 Months Ended
	Jun. 30, 2017	Jun. 30, 2016
Cash flows from operating activities:
Net income	$ 39,238	$ 38,398
Adjustments to reconcile net income to net cash provided by operating activities:
Depreciation	109,041	101,068
Deferred income taxes	6,252	23,621
Gain on disposal of property and equipment	(3,854)	(10,147)
Non-cash equity compensation	2,031	1,011
Insurance and claims accruals, net of current portion	(3,265)	(8,470)
Other	(8,206)	(5,212)
Changes in certain working capital items:
Accounts receivable, net	10,970	11,915
Other current assets	18,177	5,711
Accounts payable	1,759	3,627
Other current liabilities	318	9,500
Net cash provided by operating activities	172,461	171,022
Cash flows from investing activities:
Additions to property and equipment	(126,051)	(313,080)
Proceeds from sales of property and equipment	60,076	52,554
Decrease in notes receivable	11,490	8,297
Net cash used in investing activities	(54,485)	(252,229)
Cash flows from financing activities:
Repayments of short-term debt	(45,000)	0
Repayments of long-term debt	(60,000)	0
Proceeds from issuance of long-term debt	0	70,000
Change in net checks issued in excess of cash balances	0	6,941
Dividends on common stock	(8,663)	(8,643)
Tax withholding related to net share settlements of restricted stock awards	(360)	(559)
Stock options exercised	988	185
Excess tax benefits from equity compensation	0	(16)
Payment of notes payable	0	(3,117)
Net cash used in financing activities	(113,035)	64,791
Effect of exchange rate fluctuations on cash	707	(180)
Net increase (decrease) in cash and cash equivalents	5,648	(16,596)
Cash and cash equivalents, beginning of period	16,962	31,833
Cash and cash equivalents, end of period	22,610	15,237
Supplemental disclosures of cash flow information:
Interest paid	1,512	1,072
Income taxes paid	4,468	4,238
Supplemental schedule of non-cash investing activities:
Notes receivable issued upon sale of property and equipment	2,599	18,743
Change in fair value of interest rate swap	234	(905)
Property and equipment acquired included in accounts payable	4,214	11,904
Property and equipment disposed included in other receivables	$ 103	$ 606