Consolidated Statements of Comprehensive Income - USD ($) shares in Thousands, $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Income Statement [Abstract]
Operating Revenue	$ 151,279	$ 182,114	$ 463,800	$ 441,632
Operating Expenses
Salaries, wages, and benefits	55,126	71,399	174,694	169,020
Rent and purchased transportation	4,067	16,619	15,652	21,301
Fuel	27,460	29,739	85,340	73,731
Operating Costs and Expenses	6,469	9,122	20,970	21,951
Cost, Direct Tax and License	3,938	5,410	12,039	11,845
Insurance and claims	4,407	5,979	12,862	13,339
Communications and utilities	1,416	1,487	4,852	3,623
Depreciation and amortization	25,133	28,784	75,490	74,318
Other operating expenses	5,287	8,047	17,083	18,674
Gain on disposal of property and equipment	(7,156)	(7,471)	(15,410)	(19,845)
Total operating expenses	126,147	169,115	403,572	387,957
Operating income	25,132	12,999	60,228	53,675
Interest income	586	238	1,351	950
Interest expense	0	(175)	0	(175)
Income before income taxes	25,718	13,062	61,579	54,450
Federal and state income taxes	6,662	5,146	11,342	17,882
Net income	19,056	7,916	50,237	36,568
Other comprehensive income, net of tax	0	0	0	0
Comprehensive income	$ 19,056	$ 7,916	$ 50,237	$ 36,568
Net income per share
Basic	$ 0.23	$ 0.10	$ 0.61	$ 0.44
Diluted	$ 0.23	$ 0.09	$ 0.61	$ 0.44
Weighted average shares outstanding
Basic	81,965	83,303	82,530	83,296
Diluted	81,992	83,333	82,564	83,336
Dividends declared per share	$ 0.02	$ 0.02	$ 0.06	$ 0.06