Condensed Consolidated Statements of Operations (Unaudited) - USD ($) shares in Thousands, $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016
Income Statement [Abstract]
Operating Revenue	$ 350,062	$ 316,442	$ 1,025,259	$ 918,258
Operating Expenses:
Salaries, wages and employees’ benefits	194,920	178,687	572,211	524,877
Purchased transportation	23,074	15,657	60,212	42,439
Fuel, operating expenses and supplies	66,679	59,345	196,761	172,411
Operating taxes and licenses	10,631	10,061	32,088	30,227
Claims and insurance	8,535	9,988	28,010	28,949
Depreciation and amortization	22,338	19,927	64,607	56,910
Loss (gain) from property disposals, net	(717)	133	(469)	496
Total operating expenses	325,460	293,798	953,420	856,309
Operating Income	24,602	22,644	71,839	61,949
Nonoperating Expenses (Income):
Interest expense	1,313	1,183	3,762	3,410
Other, net	(131)	(104)	57	(147)
Nonoperating expenses, net	1,182	1,079	3,819	3,263
Income Before Income Taxes	23,420	21,565	68,020	58,686
Income Tax Provision	9,013	7,739	24,623	21,010
Net Income	$ 14,407	$ 13,826	$ 43,397	$ 37,676
Weighted average common shares outstanding – basic	25,527	25,038	25,494	25,022
Weighted average common shares outstanding – diluted	26,113	25,658	26,050	25,625
Basic Earnings Per Share	$ 0.56	$ 0.55	$ 1.70	$ 1.51
Diluted Earnings Per Share	$ 0.55	$ 0.54	$ 1.67	$ 1.47