Condensed Consolidated Statements of Earnings (Unaudited) - USD ($) shares in Thousands, $ in Thousands	3 Months Ended
	Mar. 31, 2019	Mar. 31, 2018
Operating revenues	$ 2,089,627	$ 1,948,245
Operating expenses:
Rents and purchased transportation	999,889	964,892
Salaries, wages and employee benefits	516,326	450,265
Depreciation and amortization	119,930	105,583
Fuel and fuel taxes	112,125	107,881
Operating supplies and expenses	78,172	70,681
General and administrative expenses, net of asset dispositions	45,038	32,326
Insurance and claims	28,994	28,499
Operating taxes and licenses	13,160	11,588
Communication and utilities	8,198	7,749
Total operating expenses	1,921,832	1,779,464
Operating income	167,795	168,781
Net interest expense	13,033	9,152
Earnings before income taxes	154,762	159,629
Income taxes	35,161	41,487
Net earnings	$ 119,601	$ 118,142
Weighted average basic shares outstanding (in shares)	108,730	109,754
Basic earnings per share (in dollars per share)	$ 1.10	$ 1.08
Weighted average diluted shares outstanding (in shares)	109,664	110,863
Diluted earnings per share (in dollars per share)	$ 1.09	$ 1.07
Dividends declared per common share (in dollars per share)	$ 0.26	$ 0.24
Service Excluding Fuel Surcharge [Member]
Operating revenues	$ 1,855,341	$ 1,712,934
Fuel Surcharge [Member]
Operating revenues	$ 234,286	$ 235,311