Condensed Consolidated Statements of Operations (Unaudited) - USD ($) shares in Thousands, $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019
Income Statement [Abstract]
Operating Revenue	$ 481,374	$ 468,891	$ 1,345,884	$ 1,343,670
Operating Expenses:
Salaries, wages and employees' benefits	252,092	250,162	715,014	708,203
Purchased transportation	40,053	35,843	96,518	98,415
Fuel, operating expenses and supplies	74,106	84,259	222,907	253,130
Operating taxes and licenses	14,061	13,634	42,200	40,365
Claims and insurance	11,938	7,850	40,652	30,536
Depreciation and amortization	34,224	31,333	100,478	87,258
Loss (gain) from property disposals, net	(316)	451	(1,558)	607
Total operating expenses	426,158	423,532	1,216,211	1,218,514
Operating Income	55,216	45,359	129,673	125,156
Nonoperating Expenses (Income):
Interest expense	1,174	1,868	4,170	5,154
Other, net	(391)	(20)	(595)	(494)
Nonoperating expenses, net	783	1,848	3,575	4,660
Income Before Income Taxes	54,433	43,511	126,098	120,496
Income Tax Provision	12,894	10,543	27,994	28,196
Net Income	$ 41,539	$ 32,968	$ 98,104	$ 92,300
Weighted average common shares outstanding – basic	26,150	25,978	26,118	25,936
Weighted average common shares outstanding – diluted	26,615	26,460	26,569	26,413
Basic Earnings Per Share	$ 1.59	$ 1.27	$ 3.76	$ 3.56
Diluted Earnings Per Share	$ 1.56	$ 1.25	$ 3.69	$ 3.49