Condensed Consolidated Statements of Earnings (Unaudited) - USD ($) shares in Thousands, $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2015	Sep. 30, 2014	Sep. 30, 2015	Sep. 30, 2014
Operating revenues, excluding fuel surcharge revenues	$ 1,419,451	$ 1,316,951	$ 4,043,991	$ 3,737,936
Fuel surcharge revenues	167,043	284,205	522,640	817,994
Total operating revenues	1,586,494	1,601,156	4,566,631	4,555,930
Operating expenses:
Rents and purchased transportation	770,148	802,092	2,194,683	2,282,992
Salaries, wages and employee benefits	351,651	327,106	1,030,438	951,517
Fuel and fuel taxes	76,755	115,503	243,459	352,452
Depreciation and amortization	86,201	75,416	251,239	216,109
Operating supplies and expenses	57,299	56,978	164,497	165,939
Insurance and claims	20,078	21,904	55,714	57,622
General and administrative expenses, net of asset dispositions	14,866	15,057	56,471	36,525
Operating taxes and licenses	10,683	9,893	31,505	28,517
Communication and utilities	4,967	5,107	15,824	15,620
Total operating expenses	1,392,648	1,429,056	4,043,830	4,107,293
Operating income	193,846	172,100	522,801	448,637
Net interest expense	7,838	6,649	21,202	21,358
Earnings before income taxes	186,008	165,451	501,599	427,279
Income taxes	70,869	63,037	191,109	162,793
Net earnings	$ 115,139	$ 102,414	$ 310,490	$ 264,486
Weighted average basic shares outstanding (in shares)	115,456	117,050	116,157	117,020
Basic earnings per share (in dollars per share)	$ 1	$ 0.87	$ 2.67	$ 2.26
Weighted average diluted shares outstanding (in shares)	116,282	118,221	117,293	118,567
Diluted earnings per share (in dollars per share)	$ 0.99	$ 0.87	$ 2.65	$ 2.23
Dividends declared per common share (in dollars per share)	$ 0.21	$ 0.20	$ 0.63	$ 0.60