Condensed Statements of Operations (Unaudited) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2021	Jun. 30, 2020	Jun. 30, 2021	Jun. 30, 2020
Income Statement [Abstract]
Revenue from operations	$ 1,319,409	$ 896,210	$ 2,445,924	$ 1,883,574
Operating expenses:
Salaries, wages and benefits	611,518	460,906	1,157,177	985,389
Operating supplies and expenses	137,577	75,412	261,733	183,105
General supplies and expenses	34,427	25,881	65,595	59,489
Operating taxes and licenses	33,256	27,043	64,522	56,357
Insurance and claims	14,983	10,910	27,905	20,760
Communications and utilities	8,390	7,262	16,586	15,453
Depreciation and amortization	63,947	65,735	127,934	131,170
Purchased transportation	43,927	18,983	78,641	39,783
Miscellaneous expenses, net	5,340	4,912	10,130	9,732
Total operating expenses	953,365	697,044	1,810,223	1,501,238
Operating income	366,044	199,166	635,701	382,336
Non-operating expense (income):
Interest expense	465	765	972	865
Interest income	(196)	(231)	(482)	(1,479)
Other expense (income), net	1,387	(373)	1,515	3,244
Total non-operating expense	1,656	161	2,005	2,630
Income before income taxes	364,388	199,005	633,696	379,706
Provision for income taxes	94,812	51,200	164,761	98,724
Net income	$ 269,576	$ 147,805	$ 468,935	$ 280,982
Earnings per share:
Basic	$ 2.33	$ 1.26	$ 4.04	$ 2.37
Diluted	$ 2.31	$ 1.25	$ 4.01	$ 2.36
Weighted average shares outstanding:
Basic	115,820,522	117,610,178	116,157,336	118,330,176
Diluted	116,561,242	118,359,884	116,906,572	119,082,728
Dividends declared per share	$ 0.20	$ 0.15	$ 0.40	$ 0.30