Condensed Consolidated Statements of Operations (Unaudited) - USD ($) shares in Thousands, $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2019	Sep. 30, 2018
Income Statement [Abstract]
Operating Revenue	$ 468,891	$ 425,562	$ 1,343,670	$ 1,247,099
Operating Expenses:
Salaries, wages and employees’ benefits	250,162	224,635	708,203	656,165
Purchased transportation	35,843	31,216	98,415	95,245
Fuel, operating expenses and supplies	84,259	81,643	253,130	245,182
Operating taxes and licenses	13,634	12,366	40,365	37,310
Claims and insurance	7,850	9,985	30,536	30,086
Depreciation and amortization	31,333	26,694	87,258	74,965
Loss from property disposals, net	451	326	607	305
Total operating expenses	423,532	386,865	1,218,514	1,139,258
Operating Income	45,359	38,697	125,156	107,841
Nonoperating Expenses (Income):
Interest expense	1,868	1,410	5,154	4,090
Other, net	(20)	(139)	(494)	(384)
Nonoperating expenses, net	1,848	1,271	4,660	3,706
Income Before Income Taxes	43,511	37,426	120,496	104,135
Income Tax Provision	10,543	9,231	28,196	24,534
Net Income	$ 32,968	$ 28,195	$ 92,300	$ 79,601
Weighted average common shares outstanding – basic	25,978	25,792	25,936	25,752
Weighted average common shares outstanding – diluted	26,460	26,354	26,413	26,328
Basic Earnings Per Share	$ 1.27	$ 1.09	$ 3.56	$ 3.09
Diluted Earnings Per Share	$ 1.25	$ 1.07	$ 3.49	$ 3.02