Condensed Statements of Operations (Unaudited) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019
Income Statement [Abstract]
Revenue from operations	$ 1,058,166	$ 1,048,457	$ 2,941,740	$ 3,099,905
Operating expenses:
Salaries, wages and benefits	524,658	533,451	1,510,047	1,588,378
Operating supplies and expenses	90,269	117,343	273,374	361,110
General supplies and expenses	27,383	33,633	86,872	97,584
Operating taxes and licenses	29,923	29,117	86,280	87,572
Insurance and claims	11,821	11,280	32,581	34,039
Communications and utilities	7,622	8,098	23,075	22,071
Depreciation and amortization	64,983	63,493	196,153	189,137
Purchased transportation	25,405	23,063	65,188	68,218
Miscellaneous expenses, net	5,858	11,452	15,590	21,354
Total operating expenses	787,922	830,930	2,289,160	2,469,463
Operating income	270,244	217,527	652,580	630,442
Non-operating expense (income):
Interest expense	1,071	3	1,936	285
Interest income	(123)	(1,714)	(1,602)	(4,966)
Other expense, net	961	844	4,205	768
Total non-operating expense (income)	1,909	(867)	4,539	(3,913)
Income before income taxes	268,335	218,394	648,041	634,355
Provision for income taxes	66,467	54,295	165,191	162,861
Net income	$ 201,868	$ 164,099	$ 482,850	$ 471,494
Earnings per share:
Basic	$ 1.72	$ 1.37	$ 4.09	$ 3.91
Diluted	$ 1.71	$ 1.37	$ 4.07	$ 3.90
Weighted average shares outstanding:
Basic	117,188,398	119,820,422	117,946,805	120,711,513
Diluted	117,933,440	119,984,153	118,696,836	120,882,836
Dividends declared per share	$ 0.15	$ 0.11	$ 0.45	$ 0.34