Consolidated Statements of Comprehensive Income - USD ($) shares in Thousands, $ in Thousands	3 Months Ended
	Mar. 31, 2017	Mar. 31, 2016
Income Statement [Abstract]
Operating Revenue	$ 129,903	$ 162,786
Operating Expenses
Salaries, wages, and benefits	48,979	65,466
Rent and purchased transportation	2,863	6,700
Fuel	22,702	21,194
Operations and maintenance	5,869	6,639
Operating taxes and licenses	3,292	3,891
Insurance and claims	3,779	8,092
Communications and utilities	1,098	1,204
Depreciation and amortization	22,930	25,705
Other operating expenses	5,103	4,933
Gain on disposal of property and equipment	(6,075)	(1,288)
Total operating expenses	110,540	142,536
Operating income	19,363	20,250
Interest income	288	75
Income before income taxes	19,651	20,325
Federal and state income taxes	5,615	5,948
Net income	14,036	14,377
Other comprehensive income, net of tax	0	0
Comprehensive income	$ 14,036	$ 14,377
Net income per share
Basic	$ 0.17	$ 0.17
Diluted	$ 0.17	$ 0.17
Weighted average shares outstanding
Basic	83,292	83,369
Diluted	83,337	83,460
Dividends declared per share	$ 0.02	$ 0.02