Consolidated Statements of Operations - USD ($) shares in Thousands, $ in Thousands	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Income Statement [Abstract]
Operating Revenue	$ 1,218,481	$ 1,221,311	$ 1,272,321
Operating Expenses:
Salaries, wages and employees’ benefits	696,046	670,173	639,633
Purchased transportation	56,329	70,611	99,610
Fuel, operating expenses and supplies	230,364	261,387	314,788
Operating taxes and licenses	40,025	37,003	36,028
Claims and insurance	39,625	26,832	37,563
Depreciation and amortization	76,240	65,020	59,022
Operating (gains) losses, net	716	310	(16)
Total operating expenses	1,139,345	1,131,336	1,186,628
Operating Income	79,136	89,975	85,693
Nonoperating Expenses (Income):
Interest expense	4,394	4,107	4,564
Other, net	(177)	(95)	(99)
Nonoperating expenses, net	4,217	4,012	4,465
Income Before Income Taxes	74,919	85,963	81,228
Income Tax Expense	26,895	30,947	29,237
Net Income	$ 48,024	$ 55,016	$ 51,991
Weighted average common shares outstanding – basic	25,038	24,919	24,505
Weighted average common shares outstanding – diluted	25,680	25,471	25,463
Basic Earnings Per Share	$ 1.92	$ 2.21	$ 2.12
Diluted Earnings Per Share	$ 1.87	$ 2.16	$ 2.04