Condensed Consolidated Statements of Operations (unaudited) - USD ($) shares in Thousands, $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2024	Jun. 30, 2023	Jun. 30, 2024	Jun. 30, 2023
Income Statement [Abstract]
Operating Revenue	$ 823,244	$ 694,622	$ 1,578,019	$ 1,355,157
Operating Expenses:
Salaries, wages and employees' benefits	372,240	311,888	713,953	610,844
Purchased transportations	61,047	49,771	113,554	96,498
Fuel, operating expenses and supplies	160,877	133,490	317,202	275,115
Operating taxes and licenses	19,693	17,457	39,459	34,522
Claims and insurance	18,828	16,956	36,291	31,015
Depreciation and amortization	52,536	44,658	101,385	87,538
Other operating, net	430	147	670	227
Total operating expenses	685,651	574,367	1,322,514	1,135,759
Operating Income	137,593	120,255	255,505	219,398
Nonoperating (Income) Expenses:
Interest expense	2,412	458	2,954	1,146
Interest income	(110)	(487)	(865)	(627)
Other, net	(326)	(990)	(1,114)	(1,493)
Nonoperating (income) expenses, net	1,976	(1,019)	975	(974)
Income Before Income Taxes	135,617	121,274	254,530	220,372
Income Tax Provision	33,098	29,955	61,316	52,956
Net Income	$ 102,519	$ 91,319	$ 193,214	$ 167,416
Weighted average common shares outstanding - basic	26,691	26,634	26,682	26,617
Weighted average common shares outstanding - diluted	26,802	26,736	26,798	26,722
Basic Earnings Per Share	$ 3.84	$ 3.43	$ 7.24	$ 6.29
Diluted Earnings Per Share	$ 3.83	$ 3.42	$ 7.21	$ 6.27