Consolidated Statements of Comprehensive Income - USD ($) shares in Thousands, $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2015	Jun. 30, 2014	Jun. 30, 2015	Jun. 30, 2014
Income Statement [Abstract]
Operating Revenue	$ 191,684	$ 226,785	$ 379,207	$ 451,265
Operating Expenses
Salaries, wages, and benefits	70,904	71,240	141,900	142,185
Rent and purchased transportation	9,211	13,741	18,537	28,252
Fuel	34,196	59,338	68,452	122,563
Operations and maintenance	8,379	9,776	16,512	19,897
Operating taxes and licenses	4,378	5,319	9,192	10,165
Insurance and claims	3,469	4,370	10,113	11,465
Communications and utilities	1,453	1,553	2,996	3,383
Depreciation and amortization	26,876	26,668	52,850	51,241
Other operating expenses	6,747	7,997	14,505	16,687
Gain on disposal of property and equipment	(9,668)	(13,859)	(19,849)	(15,903)
Total operating expenses	155,945	186,143	315,208	389,935
Operating income	35,739	40,642	63,999	61,330
Interest income	61	106	93	142
Interest expense	0	(132)	(19)	(287)
Income before income taxes	35,800	40,616	64,073	61,185
Federal and state income taxes	12,484	14,144	23,145	20,634
Net income	23,316	26,472	40,928	40,551
Other comprehensive income, net of tax	0	0	0	0
Comprehensive income	$ 23,316	$ 26,472	$ 40,928	$ 40,551
Net income per share
Basic	$ 0.27	$ 0.30	$ 0.47	$ 0.46
Diluted	$ 0.27	$ 0.30	$ 0.47	$ 0.46
Weighted average shares outstanding
Basic	87,814	87,728	87,802	87,716
Diluted	87,967	87,900	87,966	87,908
Dividends declared per share	$ 0.02	$ 0.02	$ 0.04	$ 0.04