Condensed Statements of Operations (Unaudited) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2022	Sep. 30, 2021	Sep. 30, 2022	Sep. 30, 2021
Income Statement [Abstract]
Revenue from operations	$ 1,603,690	$ 1,400,046	$ 4,768,418	$ 3,845,970
Operating expenses:
Salaries, wages and benefits	675,084	649,685	2,060,983	1,806,862
Operating supplies and expenses	217,260	146,509	645,329	408,242
General supplies and expenses	45,951	37,312	120,580	102,907
Operating taxes and licenses	35,753	34,028	105,781	98,550
Insurance and claims	17,491	15,674	50,562	43,579
Communications and utilities	10,288	8,738	30,062	25,324
Depreciation and amortization	68,347	65,160	203,997	193,094
Purchased transportation	34,453	52,037	129,634	130,678
Miscellaneous expenses, net	2,983	7,493	11,087	17,623
Total operating expenses	1,107,610	1,016,636	3,358,015	2,826,859
Operating income	496,080	383,410	1,410,403	1,019,111
Non-operating expense (income):
Interest expense	997	371	1,283	1,343
Interest income	(1,333)	(178)	(1,933)	(660)
Other expense, net	351	266	1,706	1,781
Total non-operating expense	15	459	1,056	2,464
Income before income taxes	496,065	382,951	1,409,347	1,016,647
Provision for income taxes	118,664	96,317	356,117	261,078
Net income	$ 377,401	$ 286,634	$ 1,053,230	$ 755,569
Earnings per share:
Basic	$ 3.38	$ 2.48	$ 9.32	$ 6.52
Diluted	$ 3.36	$ 2.47	$ 9.26	$ 6.48
Weighted average shares outstanding:
Basic	111,569,260	115,411,032	113,011,703	115,905,834
Diluted	112,294,938	116,161,555	113,746,887	116,655,504
Dividends declared per share	$ 0.30	$ 0.20	$ 0.90	$ 0.60