Consolidated Statements of Cash Flows (USD $) In Thousands, unless otherwise specified	3 Months Ended
	Mar. 31, 2015	Mar. 31, 2014
OPERATING ACTIVITIES
Net income	$ 17,611	$ 14,079
Adjustments to reconcile net income to net cash provided by operating activities:
Depreciation and amortization	26,237	24,849
Deferred income taxes	(1,524)	3,716
Amortization of stock-based compensation	287	261
Gain on disposal of property and equipment	(10,181)	(2,043)
Changes in certain working capital items:
Trade receivables	3,290	(6,765)
Prepaid expenses and other current assets	1,475	(1,659)
Accounts payable, accrued liabilities, and accrued expenses	6,805	(2,802)
Accrued income taxes	11,690	2,353
Net cash provided by operating activities	55,690	31,989
INVESTING ACTIVITIES
Proceeds from sale of property and equipment	25,547	4,840
Purchases of property and equipment, net of trades	(5,222)	(21,560)
Acquisition of business, net of cash acquired	0	(3,011)
Change in designated funds for equipment purchases	(18,205)	0
Change in other assets	1,658	(90)
Net cash provided by (used in) investing activities	3,778	(19,821)
FINANCING ACTIVITIES
Proceeds from Lines of Credit	0	3,000
Repayments on line of credit	(24,600)	(16,000)
Net cash used in financing activities	(24,600)	(13,000)
Net increase (decrease) in cash and cash equivalents	34,868	(832)
CASH AND CASH EQUIVALENTS
Beginning of period	17,303	17,763
End of period	52,171	16,931
SUPPLEMENTAL DISCLOSURES OF CASH FLOW INFORMATION
Interest paid	40	163
Cash paid during the period for income taxes, net of refunds	492	421
Noncash investing and financing activities:
Fair value of revenue equipment traded	0	3,198
Purchased property and equipment in accounts payable	20,164	10,142
Sold property and equipment in other current assets	6,882	0
Common stock dividends declared in accounts payable	$ 1,759	$ 1,758