Statements of Operations - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Income Statement [Abstract]
Revenue from operations	$ 4,015,129	$ 4,109,111	$ 4,043,695
Operating expenses:
Salaries, wages and benefits	2,053,894	2,122,464	2,075,602
Operating supplies and expenses	373,431	473,114	497,476
General supplies and expenses	110,279	123,975	119,180
Operating taxes and licenses	116,943	116,839	112,210
Insurance and claims	42,364	52,549	44,118
Communications and utilities	31,542	29,601	31,070
Depreciation and amortization	261,259	253,681	230,357
Purchased transportation	97,947	89,636	96,017
Miscellaneous expenses, net	20,588	28,546	20,614
Total operating expenses	3,108,247	3,290,405	3,226,644
Operating income	906,882	818,706	817,051
Non-operating expense (income):
Interest expense	2,782	377	189
Interest income	(1,830)	(6,763)	(3,113)
Other expense, net	4,566	1,143	4,462
Total non-operating expense (income)	5,518	(5,243)	1,538
Income before income taxes	901,364	823,949	815,513
Provision for income taxes	228,682	208,431	209,845
Net income	$ 672,682	$ 615,518	$ 605,668
Earnings per share:
Basic	$ 5.71	$ 5.11	$ 4.93
Diluted	$ 5.68	$ 5.10	$ 4.92
Weighted average shares outstanding:
Basic	117,737,180	120,414,218	122,885,346
Diluted	118,493,203	120,609,599	123,029,672
Dividends declared per share	$ 0.60	$ 0.45	$ 0.35