Consolidated Statements of Operations - USD ($) shares in Thousands, $ in Thousands	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Income Statement [Abstract]
Operating Revenue	$ 1,221,311	$ 1,272,321	$ 1,139,094
Operating Expenses:
Salaries, wages and employees’ benefits	670,173	639,633	572,487
Purchased transportation	70,611	99,610	72,975
Fuel, operating expenses and supplies	261,387	314,788	306,364
Operating taxes and licenses	37,003	36,028	36,513
Claims and insurance	26,832	37,563	25,494
Depreciation and amortization	65,020	59,022	51,564
Operating (gains) losses, net	310	(16)	(721)
Total operating expenses	1,131,336	1,186,628	1,064,676
Operating Income	89,975	85,693	74,418
Nonoperating Expenses (Income):
Interest expense	4,107	4,564	6,490
Other, net	(95)	(99)	(217)
Nonoperating expenses, net	4,012	4,465	6,273
Income Before Income Taxes	85,963	81,228	68,145
Income Tax Expense	30,947	29,237	24,518
Net Income	$ 55,016	$ 51,991	$ 43,627
Weighted average common shares outstanding – basic	24,919	24,505	24,154
Weighted average common shares outstanding – diluted	25,471	25,463	25,205
Basic Earnings Per Share	$ 2.21	$ 2.12	$ 1.81
Diluted Earnings Per Share	$ 2.16	$ 2.04	$ 1.73