Consolidated Statements of Comprehensive Income - USD ($) shares in Thousands, $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
OPERATING REVENUE	$ 607,336	$ 612,937	$ 736,345
Operating Expenses
Salaries, wages and benefits	236,872	231,980	277,318
Rent and purchased transportation	30,002	23,485	34,489
Fuel	104,381	91,494	123,714
Operations and maintenance	29,609	26,159	34,025
Operating taxes and licenses	16,615	15,559	18,095
Insurance and claims	18,850	24,449	21,618
Communications and utilities	5,781	4,485	6,001
Depreciation and amortization	103,690	105,578	110,973
Other operating expenses	24,666	13,385	28,572
Gain on disposal of property and equipment	(26,674)	(9,205)	(35,040)
Total operating expenses	543,792	527,369	619,765
Operating income	63,544	85,568	116,580
Interest income	1,129	481	210
Interest expense	(175)	0	(19)
Income before income taxes	64,498	86,049	116,771
Federal and state income tax (benefit) expense	(10,675)	29,663	43,715
Net income	75,173	56,386	73,056
Other comprehensive income, net of tax	0	0	0
Comprehensive income	$ 75,173	$ 56,386	$ 73,056
Net income per share
Basic	$ 0.90	$ 0.68	$ 0.84
Diluted	$ 0.90	$ 0.68	$ 0.84
Weighted average shares outstanding
Basic	83,298	83,297	86,974
Diluted	83,336	83,365	87,109
Dividends declared per share	$ 0.08	$ 0.08	$ 0.08