Condensed Consolidated Statements of Earnings (Unaudited) - USD ($) shares in Thousands, $ in Thousands	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017
Operating revenues	$ 1,948,245	$ 1,629,158
Operating expenses:
Rents and purchased transportation	964,892	806,439
Salaries, wages and employee benefits	450,265	380,311
Fuel and fuel taxes	107,881	80,646
Depreciation and amortization	105,583	92,189
Operating supplies and expenses	70,681	58,022
General and administrative expenses, net of asset dispositions	32,326	23,481
Insurance and claims	28,499	23,005
Operating taxes and licenses	11,588	10,680
Communication and utilities	7,749	4,996
Total operating expenses	1,779,464	1,479,769
Operating income	168,781	149,389
Net interest expense	9,152	6,817
Earnings before income taxes	159,629	142,572
Income taxes	41,487	39,870
Net earnings	$ 118,142	$ 102,702
Weighted average basic shares outstanding (in shares)	109,754	110,878
Basic earnings per share (in dollars per share)	$ 1.08	$ 0.93
Weighted average diluted shares outstanding (in shares)	110,863	112,026
Diluted earnings per share (in dollars per share)	$ 1.07	$ 0.92
Dividends declared per common share (in dollars per share)	$ 0.24	$ 0.23
Service Excluding Fuel Surcharge [Member]
Operating revenues	$ 1,712,934	$ 1,461,768
Fuel Surcharge [Member]
Operating revenues	$ 235,311	$ 167,390