Consolidated Statements of Cash Flows - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2017	Mar. 31, 2016
OPERATING ACTIVITIES
Net income	$ 14,036	$ 14,377
Adjustments to reconcile net income to net cash provided by operating activities:
Depreciation and amortization	22,940	25,705
Deferred income taxes	53	(6,778)
Stock-based compensation	113	0
Amortization of stock-based compensation, net of tax	0	587
Gain on disposal of property and equipment	(6,075)	(1,288)
Changes in certain working capital items:
Trade receivables	623	2,512
Prepaid expenses and other current assets	(3,351)	1,666
Accounts payable, accrued liabilities, and accrued expenses	(1,814)	6,599
Accrued income taxes	5,574	12,530
Net cash provided by operating activities	32,099	55,910
INVESTING ACTIVITIES
Proceeds from sale of property and equipment	21,542	3,783
Purchases of property and equipment, net of trades	(17,617)	(2,865)
Change in designated funds for equipment purchases	(3,823)	(3,875)
Change in other assets	192	(94)
Net cash provided by (used in) investing activities	294	(3,051)
FINANCING ACTIVITIES
Payments of cash dividends	(1,667)	(1,666)
Shares withheld for employee taxes related to stock-based compensation	(63)	0
Repurchases of common stock	0	(14,678)
Net cash used in financing activities	(1,730)	(16,344)
Net increase in cash and cash equivalents	30,663	36,515
CASH AND CASH EQUIVALENTS
Beginning of period	128,507	33,232
End of period	159,170	69,747
SUPPLEMENTAL DISCLOSURES OF CASH FLOW INFORMATION
Cash (received) paid during the period for income taxes, net of refunds	(12)	221
Noncash investing and financing activities:
Purchased property and equipment in accounts payable	3,406	0
Sold revenue equipment in other current assets	$ 2,602	$ 892