Condensed Consolidated Statements of Operations (Unaudited) - USD ($) shares in Thousands, $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Income Statement [Abstract]
Operating Revenue	$ 425,562	$ 357,010	$ 1,247,099	$ 1,044,505
Operating Expenses:
Salaries, wages and employees’ benefits	224,635	194,920	656,165	572,211
Purchased transportation	31,216	30,056	95,245	79,516
Fuel, operating expenses and supplies	81,643	66,679	245,182	196,761
Operating taxes and licenses	12,366	10,631	37,310	32,088
Claims and insurance	9,985	8,535	30,086	28,010
Depreciation and amortization	26,694	22,338	74,965	64,607
Loss (gain) from property disposals, net	326	(717)	305	(469)
Total operating expenses	386,865	332,442	1,139,258	972,724
Operating Income	38,697	24,568	107,841	71,781
Nonoperating Expenses (Income):
Interest expense	1,410	1,313	4,090	3,762
Other, net	(139)	(131)	(384)	57
Nonoperating expenses, net	1,271	1,182	3,706	3,819
Income Before Income Taxes	37,426	23,386	104,135	67,962
Income Tax Provision	9,231	9,013	24,534	24,623
Net Income	$ 28,195	$ 14,373	$ 79,601	$ 43,339
Weighted average common shares outstanding – basic	25,792	25,527	25,752	25,494
Weighted average common shares outstanding – diluted	26,354	26,113	26,328	26,050
Basic Earnings Per Share	$ 1.09	$ 0.56	$ 3.09	$ 1.70
Diluted Earnings Per Share	$ 1.07	$ 0.55	$ 3.02	$ 1.67