Consolidated Statements of Comprehensive Income - USD ($) shares in Thousands, $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2017	Jun. 30, 2016	Jun. 30, 2017	Jun. 30, 2016
Income Statement [Abstract]
Operating Revenue	$ 129,616	$ 160,791	$ 259,518	$ 323,577
Operating Expenses
Salaries, wages, and benefits	48,642	61,524	97,621	126,990
Rent and purchased transportation	1,820	6,181	4,682	12,881
Fuel	21,289	24,394	43,991	45,588
Operations and maintenance	6,961	6,969	12,830	13,607
Operating taxes and licenses	3,143	3,943	6,435	7,834
Insurance and claims	3,581	4,979	7,361	13,072
Communications and utilities	1,038	1,060	2,136	2,265
Depreciation and amortization	22,604	25,847	45,534	51,552
Other operating expenses	5,524	5,898	10,627	10,831
Gain on disposal of property and equipment	(6,299)	(4,511)	(12,375)	(5,800)
Total operating expenses	108,303	136,284	218,842	278,820
Operating income	21,313	24,507	40,676	44,757
Interest income	424	109	713	184
Income before income taxes	21,737	24,616	41,389	44,941
Federal and state income taxes	7,121	8,248	12,736	14,196
Net income	14,616	16,368	28,653	30,745
Other comprehensive income, net of tax	0	0	0	0
Comprehensive income	$ 14,616	$ 16,368	$ 28,653	$ 30,745
Net income per share
Basic	$ 0.18	$ 0.20	$ 0.34	$ 0.37
Diluted	$ 0.18	$ 0.20	$ 0.34	$ 0.37
Weighted average shares outstanding
Basic	83,294	83,248	83,293	83,308
Diluted	83,338	83,319	83,337	83,390
Dividends declared per share	$ 0.02	$ 0.02	$ 0.04	$ 0.04