Condensed Consolidated Statements of Earnings (Unaudited) - USD ($) shares in Thousands, $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2019	Jun. 30, 2018	Jun. 30, 2019	Jun. 30, 2018
Operating revenues	$ 2,261,647	$ 2,139,027	$ 4,351,274	$ 4,087,272
Operating expenses:
Rents and purchased transportation	1,086,416	1,073,164	2,086,305	2,038,057
Salaries, wages and employee benefits	535,147	465,326	1,051,472	915,592
Depreciation and amortization	123,995	107,423	243,924	213,006
Fuel and fuel taxes	118,037	115,541	230,162	223,422
Operating supplies and expenses	84,264	76,446	162,436	147,126
General and administrative expenses, net of asset dispositions	46,539	37,306	91,578	69,630
Insurance and claims	52,013	28,371	81,007	56,870
Operating taxes and licenses	13,565	12,234	26,725	23,822
Communication and utilities	8,578	8,404	16,777	16,153
Total operating expenses	2,068,554	1,924,215	3,990,386	3,703,678
Operating income	193,093	214,812	360,888	383,594
Net interest expense	14,763	9,855	27,796	19,008
Earnings before income taxes	178,330	204,957	333,092	364,586
Income taxes	44,697	53,305	79,858	94,792
Net earnings	$ 133,633	$ 151,652	$ 253,234	$ 269,794
Weighted average basic shares outstanding (in shares)	107,387	109,449	108,055	109,601
Basic earnings per share (in dollars per share)	$ 1.24	$ 1.39	$ 2.34	$ 2.46
Weighted average diluted shares outstanding (in shares)	108,373	110,682	109,015	110,771
Diluted earnings per share (in dollars per share)	$ 1.23	$ 1.37	$ 2.32	$ 2.44
Dividends declared per common share (in dollars per share)	$ 0.26	$ 0.24	$ 0.52	$ 0.48
Service Excluding Fuel Surcharge [Member]
Operating revenues	$ 1,996,266	$ 1,874,388	$ 3,851,608	$ 3,587,321
Fuel Surcharge [Member]
Operating revenues	$ 265,381	$ 264,639	$ 499,666	$ 499,951