Consolidated Statements of Operations - USD ($) shares in Thousands, $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Income Statement [Abstract]
Operating Revenue	$ 1,378,510	$ 1,218,481	$ 1,221,311
Operating Expenses:
Salaries, wages and employees’ benefits	766,790	696,046	670,173
Purchased transportation	81,482	56,329	70,611
Fuel, operating expenses and supplies	268,090	230,364	261,387
Operating taxes and licenses	43,330	40,025	37,003
Claims and insurance	37,162	39,625	26,832
Depreciation and amortization	87,102	76,240	65,020
Operating (gains) losses, net	(183)	716	310
Total operating expenses	1,283,773	1,139,345	1,131,336
Operating Income	94,737	79,136	89,975
Nonoperating Expenses (Income):
Interest expense	5,051	4,394	4,107
Other, net	(92)	(177)	(95)
Nonoperating expenses, net	4,959	4,217	4,012
Income Before Income Taxes	89,778	74,919	85,963
Income Tax Expense (Benefit)	(1,378)	26,895	30,947
Net Income	$ 91,156	$ 48,024	$ 55,016
Weighted average common shares outstanding – basic	25,518	25,038	24,919
Weighted average common shares outstanding – diluted	26,086	25,680	25,471
Basic Earnings Per Share	$ 3.57	$ 1.92	$ 2.21
Diluted Earnings Per Share	$ 3.49	$ 1.87	$ 2.16