Condensed Statements of Operations (Unaudited) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2021	Sep. 30, 2020	Sep. 30, 2021	Sep. 30, 2020
Income Statement [Abstract]
Revenue from operations	$ 1,400,046	$ 1,058,166	$ 3,845,970	$ 2,941,740
Operating expenses:
Salaries, wages and benefits	649,685	524,658	1,806,862	1,510,047
Operating supplies and expenses	146,509	90,269	408,242	273,374
General supplies and expenses	37,312	27,383	102,907	86,872
Operating taxes and licenses	34,028	29,923	98,550	86,280
Insurance and claims	15,674	11,821	43,579	32,581
Communications and utilities	8,738	7,622	25,324	23,075
Depreciation and amortization	65,160	64,983	193,094	196,153
Purchased transportation	52,037	25,405	130,678	65,188
Miscellaneous expenses, net	7,493	5,858	17,623	15,590
Total operating expenses	1,016,636	787,922	2,826,859	2,289,160
Operating income	383,410	270,244	1,019,111	652,580
Non-operating expense (income):
Interest expense	371	1,071	1,343	1,936
Interest income	(178)	(123)	(660)	(1,602)
Other expense, net	266	961	1,781	4,205
Total non-operating expense	459	1,909	2,464	4,539
Income before income taxes	382,951	268,335	1,016,647	648,041
Provision for income taxes	96,317	66,467	261,078	165,191
Net income	$ 286,634	$ 201,868	$ 755,569	$ 482,850
Earnings per share:
Basic	$ 2.48	$ 1.72	$ 6.52	$ 4.09
Diluted	$ 2.47	$ 1.71	$ 6.48	$ 4.07
Weighted average shares outstanding:
Basic	115,411,032	117,188,398	115,905,834	117,946,805
Diluted	116,161,555	117,933,440	116,655,504	118,696,836
Dividends declared per share	$ 0.20	$ 0.15	$ 0.60	$ 0.45