Condensed Consolidated Statements of Operations (unaudited) - USD ($) shares in Thousands, $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2022	Jun. 30, 2021	Jun. 30, 2022	Jun. 30, 2021
Income Statement [Abstract]
Operating Revenue	$ 745,554	$ 571,333	$ 1,406,770	$ 1,055,407
Operating Expenses:
Salaries, wages and employees' benefits	295,052	268,786	584,515	513,223
Purchased Transportations	91,819	62,481	170,067	107,512
Fuel, operating expenses and supplies	145,530	90,664	268,301	175,565
Operating taxes and licenses	15,979	14,559	32,552	28,897
Claims and insurance	14,216	17,328	24,952	28,808
Depreciation and amortization	36,944	34,659	76,896	70,031
Loss (gain) from property disposals, net	21	(69)	45	(268)
Total operating expenses	599,561	488,408	1,157,328	923,768
Operating Income	145,993	82,925	249,442	131,639
Nonoperating Expenses (Income):
Interest expense	668	834	1,360	1,686
Other, net	769	(430)	1,004	(561)
Nonoperating expenses, net	1,437	404	2,364	1,125
Income Before Income Taxes	144,556	82,521	247,078	130,514
Income Tax Provision	35,311	20,047	58,409	30,749
Net Income	$ 109,245	$ 62,474	$ 188,669	$ 99,765
Weighted average common shares outstanding - basic	26,507	26,332	26,489	26,309
Weighted average common shares outstandin - diluted	26,665	26,704	26,662	26,687
Basic Earnings Per Share	$ 4.12	$ 2.37	$ 7.12	$ 3.79
Diluted Earnings Per Share	$ 4.10	$ 2.34	$ 7.08	$ 3.74