Consolidated Statements of Comprehensive Income - USD ($) shares in Thousands, $ in Thousands	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017
Income Statement [Abstract]
Operating Revenue	$ 156,695	$ 129,903
Operating Expenses
Salaries, wages, and benefits	62,009	48,979
Rent and purchased transportation	6,125	2,863
Fuel	28,940	22,702
Operations and maintenance	7,865	5,869
Operating taxes and licenses	3,952	3,292
Insurance and claims	4,224	3,779
Communications and utilities	1,870	1,098
Depreciation and amortization	25,601	22,930
Other operating expenses	6,030	5,103
Gain on disposal of property and equipment	(2,869)	(6,075)
Total operating expenses	143,747	110,540
Operating income	12,948	19,363
Interest income	342	288
Income before income taxes	13,290	19,651
Federal and state income taxes	(88)	5,615
Net income	13,378	14,036
Other comprehensive income, net of tax	0	0
Comprehensive income	$ 13,378	$ 14,036
Net income per share
Basic	$ 0.16	$ 0.17
Diluted	$ 0.16	$ 0.17
Weighted average shares outstanding
Basic	83,309	83,292
Diluted	83,349	83,337
Dividends declared per share	$ 0.02	$ 0.02