Consolidated Statements of Cash Flows - USD ($) $ in Thousands	6 Months Ended
	Jun. 30, 2017	Jun. 30, 2016
OPERATING ACTIVITIES
Net income	$ 28,653	$ 30,745
Adjustments to reconcile net income to net cash provided by operating activities:
Depreciation and amortization	45,565	51,552
Deferred income taxes	1,189	(5,341)
Stock-based compensation	281	0
Amortization of stock-based compensation, net of tax	0	664
Gain on disposal of property and equipment	(12,375)	(5,800)
Changes in certain working capital items:
Trade receivables	3,000	4,000
Prepaid expenses and other current assets	(5,356)	308
Accounts payable, accrued liabilities, and accrued expenses	(7,650)	5,696
Accrued income taxes	(2,474)	(3,927)
Net cash provided by operating activities	50,833	77,897
INVESTING ACTIVITIES
Proceeds from sale of property and equipment	48,955	26,744
Purchases of property and equipment, net of trades	(50,213)	(42,353)
Change in designated funds for equipment purchases	(3,521)	0
Change in other assets	183	97
Net cash used in investing activities	(4,596)	(15,512)
FINANCING ACTIVITIES
Payments of cash dividends	(3,334)	(3,332)
Shares withheld for employee taxes related to stock-based compensation	(118)	0
Repurchases of common stock	0	(14,678)
Net cash used in financing activities	(3,452)	(18,010)
Net increase in cash and cash equivalents	42,785	44,375
CASH AND CASH EQUIVALENTS
Beginning of period	128,507	33,232
End of period	171,292	77,607
SUPPLEMENTAL DISCLOSURES OF CASH FLOW INFORMATION
Cash paid during the period for income taxes, net of refunds	14,023	23,384
Noncash investing and financing activities:
Purchased property and equipment in accounts payable	1,163	9,334
Sold revenue equipment in other current assets	$ 3,904	$ 2,501