Condensed Consolidated Statements of Operations (Unaudited) - USD ($) shares in Thousands, $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2021	Sep. 30, 2020	Sep. 30, 2021	Sep. 30, 2020
Income Statement [Abstract]
Operating Revenue	$ 616,216	$ 481,374	$ 1,671,623	$ 1,345,884
Operating Expenses:
Salaries, wages and employees' benefits	277,087	252,092	790,310	715,014
Purchased transportation	72,193	40,053	179,705	96,518
Fuel, operating expenses and supplies	98,834	74,106	274,399	222,907
Operating taxes and licenses	14,572	14,061	43,469	42,200
Claims and insurance	15,518	11,938	44,326	40,652
Depreciation and amortization	35,742	34,224	105,773	100,478
Gain from property disposals, net	(3,847)	(316)	(4,115)	(1,558)
Total operating expenses	510,099	426,158	1,433,867	1,216,211
Operating Income	106,117	55,216	237,756	129,673
Nonoperating Expenses (Income):
Interest expense	777	1,174	2,463	4,170
Other, net	14	(391)	(547)	(595)
Nonoperating expenses, net	791	783	1,916	3,575
Income Before Income Taxes	105,326	54,433	235,840	126,098
Income Tax Provision	25,617	12,894	56,366	27,994
Net Income	$ 79,709	$ 41,539	$ 179,474	$ 98,104
Weighted average common shares outstanding – basic	26,334	26,150	26,317	26,118
Weighted average common shares outstanding – diluted	26,713	26,615	26,699	26,569
Basic Earnings Per Share	$ 3.03	$ 1.59	$ 6.82	$ 3.76
Diluted Earnings Per Share	$ 2.98	$ 1.56	$ 6.72	$ 3.69