Condensed Consolidated Statements of Operations (Unaudited) - USD ($) shares in Thousands, $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2017	Jun. 30, 2016	Jun. 30, 2017	Jun. 30, 2016
Income Statement [Abstract]
Operating Revenue	$ 358,160	$ 311,905	$ 675,197	$ 601,816
Operating Expenses:
Salaries, wages and employees’ benefits	196,388	175,924	377,291	346,190
Purchased transportation	22,363	14,315	37,138	26,782
Fuel, operating expenses and supplies	66,092	59,026	130,082	113,066
Operating taxes and licenses	10,875	10,126	21,457	20,166
Claims and insurance	10,426	10,880	19,475	18,961
Depreciation and amortization	22,182	19,740	42,269	36,983
Loss from property disposals, net	116	173	248	363
Total operating expenses	328,442	290,184	627,960	562,511
Operating Income	29,718	21,721	47,237	39,305
Nonoperating Expenses (Income):
Interest expense	1,538	1,264	2,449	2,227
Other, net	90	(36)	188	(43)
Nonoperating expenses, net	1,628	1,228	2,637	2,184
Income Before Income Taxes	28,090	20,493	44,600	37,121
Income Tax Provision	10,487	7,218	15,610	13,271
Net Income	$ 17,603	$ 13,275	$ 28,990	$ 23,850
Weighted average common shares outstanding – basic	25,501	25,030	25,477	25,014
Weighted average common shares outstanding – diluted	26,000	25,583	25,982	25,560
Basic Earnings Per Share	$ 0.69	$ 0.53	$ 1.14	$ 0.95
Diluted Earnings Per Share	$ 0.68	$ 0.52	$ 1.12	$ 0.93