Condensed Consolidated Statements of Earnings (Unaudited) - USD ($) shares in Thousands, $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2021	Jun. 30, 2020	Jun. 30, 2021	Jun. 30, 2020
Operating revenues	$ 2,908,370	$ 2,145,573	$ 5,526,519	$ 4,426,398
Operating expenses:
Rents and purchased transportation	1,538,232	1,034,297	2,890,533	2,170,449
Salaries, wages and employee benefits	665,471	554,136	1,285,502	1,128,386
Depreciation and amortization	139,371	130,298	276,916	260,393
Fuel and fuel taxes	126,841	75,459	239,881	176,582
Operating supplies and expenses	91,019	79,134	172,717	164,732
General and administrative expenses, net of asset dispositions	47,505	44,599	92,396	89,761
Insurance and claims	35,508	30,899	73,538	63,260
Operating taxes and licenses	14,209	13,567	28,024	26,879
Communication and utilities	8,668	8,001	17,814	16,032
Total operating expenses	2,666,824	1,970,390	5,077,321	4,096,474
Operating income	241,546	175,183	449,198	329,924
Net interest expense	12,059	12,818	24,084	24,854
Earnings before income taxes	229,487	162,365	425,114	305,070
Income taxes	57,325	40,667	106,346	78,538
Net earnings	$ 172,162	$ 121,698	$ 318,768	$ 226,532
Weighted average basic shares outstanding (in shares)	105,509	105,505	105,594	105,745
Basic earnings per share (in dollars per share)	$ 1.63	$ 1.15	$ 3.02	$ 2.14
Weighted average diluted shares outstanding (in shares)	106,816	106,580	106,816	106,765
Diluted earnings per share (in dollars per share)	$ 1.61	$ 1.14	$ 2.98	$ 2.12
Service Excluding Fuel Surcharge [Member]
Operating revenues	$ 2,606,981	$ 1,986,130	$ 4,995,015	$ 4,031,824
Fuel Surcharge [Member]
Operating revenues	$ 301,389	$ 159,443	$ 531,504	$ 394,574