Consolidated Statements of Comprehensive Income - USD ($) shares in Thousands, $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2015	Sep. 30, 2014	Sep. 30, 2015	Sep. 30, 2014
Income Statement [Abstract]
Operating Revenue	$ 182,533	$ 217,092	$ 561,739	$ 668,358
Operating Expenses
Salaries, wages, and benefits	68,987	68,688	210,886	210,872
Rent and purchased transportation	8,238	12,518	26,775	40,770
Fuel	29,414	53,435	97,866	175,998
Operations and maintenance	9,213	9,977	25,725	29,874
Operating taxes and licenses	4,498	5,189	13,690	15,354
Insurance and claims	7,379	5,155	17,491	16,621
Communications and utilities	1,699	1,564	4,695	4,947
Depreciation and amortization	28,415	27,754	81,266	78,996
Other operating expenses	7,230	7,779	21,734	24,465
Gain on disposal of property and equipment	(7,401)	(11,257)	(27,250)	(27,160)
Total operating expenses	157,672	180,802	472,878	570,737
Operating income	24,861	36,290	88,861	97,621
Interest income	64	21	156	163
Interest expense	0	(97)	(19)	(384)
Income before income taxes	24,925	36,214	88,998	97,400
Federal and state income taxes	9,812	13,477	32,957	34,111
Net income	15,113	22,737	56,041	63,289
Other comprehensive income, net of tax	0	0	0	0
Comprehensive income	$ 15,113	$ 22,737	$ 56,041	$ 63,289
Net income per share
Basic	$ 0.17	$ 0.26	$ 0.64	$ 0.72
Diluted	$ 0.17	$ 0.26	$ 0.64	$ 0.72
Weighted average shares outstanding
Basic	87,387	87,778	87,663	87,737
Diluted	87,492	87,923	87,806	87,921
Dividends declared per share	$ 0.02	$ 0.02	$ 0.06	$ 0.06