Consolidated Statements of Comprehensive Income - USD ($) shares in Thousands, $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2016	Sep. 30, 2015	Sep. 30, 2016	Sep. 30, 2015
Income Statement [Abstract]
Operating Revenue	$ 149,316	$ 182,533	$ 472,893	$ 561,739
Operating Expenses
Salaries, wages, and benefits	58,351	68,987	185,342	210,886
Rent and purchased transportation	5,472	8,238	18,353	26,775
Fuel	22,987	29,414	68,575	97,866
Operations and maintenance	6,391	9,213	19,999	25,725
Operating taxes and licenses	3,889	4,498	11,722	13,690
Insurance and claims	4,536	7,379	17,607	17,491
Communications and utilities	1,156	1,699	3,420	4,695
Depreciation and amortization	27,271	28,415	78,823	81,266
Other operating expenses	824	7,230	11,655	21,734
Gain on disposal of property and equipment	(1,474)	(7,401)	(7,273)	(27,250)
Total operating expenses	129,403	157,672	408,223	472,878
Operating income	19,913	24,861	64,670	88,861
Interest income	124	64	308	156
Interest expense	0	0	0	(19)
Income before income taxes	20,037	24,925	64,978	88,998
Federal and state income taxes	7,510	9,812	21,706	32,957
Net income	12,527	15,113	43,272	56,041
Other comprehensive income, net of tax	0	0	0	0
Comprehensive income	$ 12,527	$ 15,113	$ 43,272	$ 56,041
Net income per share
Basic	$ 0.15	$ 0.17	$ 0.52	$ 0.64
Diluted	$ 0.15	$ 0.17	$ 0.52	$ 0.64
Weighted average shares outstanding
Basic	83,286	87,387	83,301	87,663
Diluted	83,342	87,492	83,373	87,806
Dividends declared per share	$ 0.02	$ 0.02	$ 0.06	$ 0.06