Consolidated Statements of Comprehensive Income - USD ($) shares in Thousands, $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
Income Statement [Abstract]
Operating Revenue	$ 155,826	$ 129,616	$ 312,521	$ 259,518
Operating Expenses
Salaries, wages, and benefits	57,558	48,642	119,568	97,621
Rent and purchased transportation	5,460	1,820	11,584	4,682
Fuel	28,941	21,289	57,880	43,991
Operating Costs and Expenses	6,637	6,961	14,501	12,830
Cost, Direct Tax and License	4,150	3,143	8,101	6,435
Insurance and claims	4,231	3,581	8,455	7,361
Communications and utilities	1,566	1,038	3,436	2,136
Depreciation and amortization	24,757	22,604	50,358	45,534
Other operating expenses	5,765	5,524	11,796	10,627
Gain on disposal of property and equipment	(5,386)	(6,299)	(8,254)	(12,375)
Total operating expenses	133,679	108,303	277,425	218,842
Operating income	22,147	21,313	35,096	40,676
Interest income	423	424	765	713
Income before income taxes	22,570	21,737	35,861	41,389
Federal and state income taxes	4,767	7,121	4,679	12,736
Net income	17,803	14,616	31,182	28,653
Other comprehensive income, net of tax	0	0	0	0
Comprehensive income	$ 17,803	$ 14,616	$ 31,182	$ 28,653
Net income per share
Basic	$ 0.22	$ 0.18	$ 0.38	$ 0.34
Diluted	$ 0.22	$ 0.18	$ 0.38	$ 0.34
Weighted average shares outstanding
Basic	82,330	83,294	82,817	83,293
Diluted	82,368	83,338	82,855	83,337
Dividends declared per share	$ 0.02	$ 0.02	$ 0.04	$ 0.04