Statements of Operations - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2021	Dec. 31, 2020	Dec. 31, 2019
Income Statement [Abstract]
Revenue from operations	$ 5,256,328	$ 4,015,129	$ 4,109,111
Operating expenses:
Salaries, wages and benefits	2,467,985	2,053,894	2,122,464
Operating supplies and expenses	567,615	373,431	473,114
General supplies and expenses	136,059	110,279	123,975
Operating taxes and licenses	133,452	116,943	116,839
Insurance and claims	53,549	42,364	52,549
Communications and utilities	34,149	31,542	29,601
Depreciation and amortization	259,883	261,259	253,681
Purchased transportation	185,785	97,947	89,636
Miscellaneous expenses, net	26,249	20,588	28,546
Total operating expenses	3,864,726	3,108,247	3,290,405
Operating income	1,391,602	906,882	818,706
Non-operating expense (income):
Interest expense	1,727	2,782	377
Interest income	(786)	(1,830)	(6,763)
Other expense, net	2,238	4,566	1,143
Total non-operating expense (income)	3,179	5,518	(5,243)
Income before income taxes	1,388,423	901,364	823,949
Provision for income taxes	354,048	228,682	208,431
Net income	$ 1,034,375	$ 672,682	$ 615,518
Earnings per share:
Basic	$ 8.94	$ 5.71	$ 5.11
Diluted	$ 8.89	$ 5.68	$ 5.10
Weighted average shares outstanding:
Basic	115,651,411	117,737,180	120,414,218
Diluted	116,409,989	118,493,203	120,609,599
Dividends declared per share	$ 0.80	$ 0.60	$ 0.45