Consolidated Statements of Operations - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Operating revenue	$ 671,144	$ 664,994	$ 672,929
Operating expenses (income):
Salaries, wages and benefits	224,827	209,455	183,464
Purchased transportation	110,719	118,030	123,017
Fuel and fuel taxes	94,075	104,582	153,931
Supplies and maintenance	43,912	43,319	41,490
Depreciation	82,445	75,323	68,243
Operating taxes and licenses	9,099	8,914	7,140
Insurance and claims	32,223	29,252	27,240
Communications and utilities	6,287	5,886	5,798
Other	19,756	18,858	15,987
	612,841	603,931	621,923
Operating income	58,303	61,063	51,006
Other	1,237	372	(667)
Income before income taxes	57,066	60,691	51,673
Provision for income taxes	23,602	24,946	21,839
Net income	$ 33,464	$ 35,745	$ 29,834
Basic earnings per common share (in dollars per share)	$ 1.03	$ 1.07	$ 0.89
Diluted earnings per common share (in dollars per share)	1.02	1.06	0.89
Dividends declared per common share (in dollars per share)	$ 0.10	$ 0.10	$ 0.10
Revenue Equipment [Member]
Operating expenses (income):
Gain on disposition of assets	$ (10,502)	$ (5,584)	$ (4,387)
Facility [Member]
Operating expenses (income):
Gain on disposition of assets		$ (4,104)