Condensed Consolidated Statements of Earnings (Unaudited) (USD $) In Thousands, except Per Share data, unless otherwise specified	3 Months Ended		6 Months Ended
	Jun. 30, 2013	Jun. 30, 2012	Jun. 30, 2013	Jun. 30, 2012
Operating revenues, excluding fuel surcharge revenues	$ 1,120,164	$ 1,005,519	$ 2,159,104	$ 1,944,202
Fuel surcharge revenues	262,694	249,611	515,340	476,850
Total operating revenues	1,382,858	1,255,130	2,674,444	2,421,052
Operating expenses:
Rents and purchased transportation	693,795	616,723	1,338,328	1,172,841
Salaries, wages and employee benefits	282,252	255,864	545,976	505,909
Fuel and fuel taxes	110,280	113,085	226,840	233,638
Depreciation and amortization	62,283	56,227	123,013	112,347
Operating supplies and expenses	49,626	45,540	94,540	85,351
Insurance and claims	12,111	12,595	24,881	23,640
General and administrative expenses, net of asset dispositions	12,425	6,356	23,409	10,350
Operating taxes and licenses	8,042	7,323	15,555	14,465
Communication and utilities	4,637	4,202	9,456	8,699
Total operating expenses	1,235,451	1,117,915	2,401,998	2,167,240
Operating income	147,407	137,215	272,446	253,812
Net interest expense	6,230	6,929	12,485	13,916
Earnings before income taxes	141,177	130,286	259,961	239,896
Income taxes	53,480	49,835	98,916	91,760
Net earnings	$ 87,697	$ 80,451	$ 161,045	$ 148,136
Weighted average basic shares outstanding (in Shares)	117,139	117,226	117,345	117,122
Basic earnings per share (in Dollars per share)	$ 0.75	$ 0.69	$ 1.37	$ 1.26
Weighted average diluted shares outstanding (in Shares)	119,472	120,027	119,647	119,888
Diluted earnings per share (in Dollars per share)	$ 0.73	$ 0.67	$ 1.35	$ 1.24
Dividends declared per common share (in Dollars per share)	$ 0.15	$ 0.14	$ 0.15	$ 0.28