Consolidated Statements of Comprehensive Income - USD ($) shares in Thousands, $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016
Income Statement [Abstract]
Operating Revenue	$ 182,114	$ 149,316	$ 441,632	$ 472,893
Operating Expenses
Salaries, wages, and benefits	71,399	58,351	169,020	185,342
Rent and purchased transportation	16,619	5,472	21,301	18,353
Fuel	29,739	22,987	73,731	68,575
Operations and maintenance	9,122	6,391	21,951	19,999
Operating taxes and licenses	5,410	3,889	11,845	11,722
Insurance and claims	5,979	4,536	13,339	17,607
Communications and utilities	1,487	1,156	3,623	3,420
Depreciation and amortization	28,784	27,271	74,318	78,823
Other operating expenses	8,047	824	18,674	11,655
Gain on disposal of property and equipment	(7,471)	(1,474)	(19,845)	(7,273)
Total operating expenses	169,115	129,403	387,957	408,223
Operating income	12,999	19,913	53,675	64,670
Interest income	238	124	950	308
Interest expense	(175)	0	(175)	0
Income before income taxes	13,062	20,037	54,450	64,978
Federal and state income taxes	5,146	7,510	17,882	21,706
Net income	7,916	12,527	36,568	43,272
Other comprehensive income, net of tax	0	0	0	0
Comprehensive income	$ 7,916	$ 12,527	$ 36,568	$ 43,272
Net income per share
Basic	$ 0.10	$ 0.15	$ 0.44	$ 0.52
Diluted	$ 0.09	$ 0.15	$ 0.44	$ 0.52
Weighted average shares outstanding
Basic	83,303	83,286	83,296	83,301
Diluted	83,333	83,342	83,336	83,373
Dividends declared per share	$ 0.02	$ 0.02	$ 0.06	$ 0.06