Condensed Consolidated Statements of Earnings (Unaudited) - USD ($) shares in Thousands, $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2020	Jun. 30, 2019	Jun. 30, 2020	Jun. 30, 2019
Operating revenues	$ 2,145,573	$ 2,261,647	$ 4,426,398	$ 4,351,274
Operating expenses:
Rents and purchased transportation	1,034,297	1,086,416	2,170,449	2,086,305
Salaries, wages and employee benefits	554,136	535,147	1,128,386	1,051,472
Depreciation and amortization	130,298	123,995	260,393	243,924
Fuel and fuel taxes	75,459	118,037	176,582	230,162
Operating supplies and expenses	79,134	84,264	164,732	162,436
General and administrative expenses, net of asset dispositions	44,599	46,539	89,761	91,578
Insurance and claims	30,899	52,013	63,260	81,007
Operating taxes and licenses	13,567	13,565	26,879	26,725
Communication and utilities	8,001	8,578	16,032	16,777
Total operating expenses	1,970,390	2,068,554	4,096,474	3,990,386
Operating income	175,183	193,093	329,924	360,888
Net interest expense	12,818	14,763	24,854	27,796
Earnings before income taxes	162,365	178,330	305,070	333,092
Income taxes	40,667	44,697	78,538	79,858
Net earnings	$ 121,698	$ 133,633	$ 226,532	$ 253,234
Weighted average basic shares outstanding (in shares)	105,505	107,387	105,745	108,055
Basic earnings per share (in dollars per share)	$ 1.15	$ 1.24	$ 2.14	$ 2.34
Weighted average diluted shares outstanding (in shares)	106,580	108,373	106,765	109,015
Diluted earnings per share (in dollars per share)	$ 1.14	$ 1.23	$ 2.12	$ 2.32
Dividends declared per common share (in dollars per share)	$ 0.27	$ 0.26	$ 0.54	$ 0.52
Service Excluding Fuel Surcharge [Member]
Operating revenues	$ 1,986,130	$ 1,996,266	$ 4,031,824	$ 3,851,608
Fuel Surcharge [Member]
Operating revenues	$ 159,443	$ 265,381	$ 394,574	$ 499,666