Consolidated Statements of Comprehensive Income - USD ($) shares in Thousands, $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2016	Jun. 30, 2015	Jun. 30, 2016	Jun. 30, 2015
Income Statement [Abstract]
Operating Revenue	$ 160,791	$ 191,684	$ 323,577	$ 379,207
Operating Expenses
Salaries, wages, and benefits	61,524	70,904	126,990	141,900
Rent and purchased transportation	6,181	9,211	12,881	18,537
Fuel	24,394	34,196	45,588	68,452
Operations and maintenance	6,969	8,379	13,607	16,512
Operating taxes and licenses	3,943	4,378	7,834	9,192
Insurance and claims	4,979	3,469	13,072	10,113
Communications and utilities	1,060	1,453	2,265	2,996
Depreciation and amortization	25,847	26,876	51,552	52,850
Other operating expenses	5,898	6,747	10,831	14,505
Gain on disposal of property and equipment	(4,511)	(9,668)	(5,800)	(19,849)
Total operating expenses	136,284	155,945	278,820	315,208
Operating income	24,507	35,739	44,757	63,999
Interest income	109	61	184	93
Interest expense	0	0	0	(19)
Income before income taxes	24,616	35,800	44,941	64,073
Federal and state income taxes	8,248	12,484	14,196	23,145
Net income	16,368	23,316	30,745	40,928
Other comprehensive income, net of tax	0	0	0	0
Comprehensive income	$ 16,368	$ 23,316	$ 30,745	$ 40,928
Net income per share
Basic	$ 0.20	$ 0.27	$ 0.37	$ 0.47
Diluted	$ 0.20	$ 0.27	$ 0.37	$ 0.47
Weighted average shares outstanding
Basic	83,248	87,814	83,308	87,802
Diluted	83,319	87,967	83,390	87,966
Dividends declared per share	$ 0.02	$ 0.02	$ 0.04	$ 0.04