Condensed Consolidated Statements of Earnings (Unaudited) - USD ($) shares in Thousands, $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019
Operating revenues	$ 2,472,523	$ 2,363,660	$ 6,898,921	$ 6,714,934
Operating expenses:
Rents and purchased transportation	1,297,333	1,196,377	3,467,782	3,282,682
Salaries, wages and employee benefits	594,162	554,872	1,722,548	1,606,344
Depreciation and amortization	132,392	126,803	392,786	370,728
Fuel and fuel taxes	87,350	114,764	263,932	344,926
Operating supplies and expenses	86,103	87,044	250,835	249,480
General and administrative expenses, net of asset dispositions	41,894	49,077	131,654	140,654
Insurance and claims	35,412	43,956	98,672	124,963
Operating taxes and licenses	13,696	14,266	40,575	40,991
Communication and utilities	8,678	8,639	24,710	25,416
Total operating expenses	2,297,020	2,195,798	6,393,494	6,186,184
Operating income	175,503	167,862	505,427	528,750
Net interest expense	11,895	12,357	36,749	40,153
Earnings before income taxes	163,608	155,505	468,678	488,597
Income taxes	38,112	37,095	116,650	116,953
Net earnings	$ 125,496	$ 118,410	$ 352,028	$ 371,644
Weighted average basic shares outstanding (in shares)	105,653	106,735	105,714	107,610
Basic earnings per share (in dollars per share)	$ 1.19	$ 1.11	$ 3.33	$ 3.45
Weighted average diluted shares outstanding (in shares)	106,798	107,692	106,776	108,569
Diluted earnings per share (in dollars per share)	$ 1.18	$ 1.10	$ 3.30	$ 3.42
Dividends declared per common share (in dollars per share)	$ 0.27	$ 0.26	$ 0.81	$ 0.78
Service Excluding Fuel Surcharge [Member]
Operating revenues	$ 2,296,053	$ 2,102,494	$ 6,327,876	$ 5,954,102
Fuel Surcharge [Member]
Operating revenues	$ 176,470	$ 261,166	$ 571,045	$ 760,832