Summary of Unaudited Results of Operations for Each Quarter (Detail) (USD $) In Thousands, except Per Share data, unless otherwise specified	3 Months Ended								12 Months Ended
	Dec. 31, 2011	Sep. 30, 2011	Jun. 30, 2011	Mar. 31, 2011	Dec. 31, 2010	Sep. 30, 2010	Jun. 30, 2010	Mar. 31, 2010	Dec. 31, 2011	Dec. 31, 2010	Dec. 31, 2009
Total revenues:
Transportation	$ 2,200,258	$ 2,280,208	$ 2,269,036	$ 1,991,022	$ 1,946,325	$ 2,026,154	$ 1,963,944	$ 1,639,236	$ 8,740,524	$ 7,575,659	$ 5,976,102
Sourcing	352,744	399,220	423,536	360,028	364,337	380,108	476,074	422,655	1,535,528	1,643,174	1,555,292
Payment Services	15,282	15,500	15,090	14,422	14,687	14,095	13,964	12,726	60,294	55,472	45,795
Total revenues	2,568,284	2,694,928	2,707,662	2,365,472	2,325,349	2,420,357	2,453,982	2,074,617	10,336,346	9,274,305	7,577,189
Costs and expenses:
Purchased transportation and related services	1,841,586	1,905,731	1,901,189	1,648,102	1,604,552	1,689,590	1,654,089	1,354,299	7,296,608	6,302,530	4,768,520
Purchased products sourced for resale	325,313	366,131	388,607	327,029	332,633	348,187	435,260	387,717	1,407,080	1,503,797	1,426,710
Personnel expenses	164,062	178,117	178,945	175,109	169,271	161,947	154,091	146,755	696,233	632,064	597,568
Other selling, general, and administrative expenses	65,368	60,984	58,826	58,517	54,828	54,300	54,087	49,839	243,695	213,054	199,580
Total costs and expenses	2,396,329	2,510,963	2,527,567	2,208,757	2,161,284	2,254,024	2,297,527	1,938,610	9,643,616	8,651,445	6,992,378
Income from operations	171,955	183,965	180,095	156,715	164,065	166,333	156,455	136,007	692,730	622,860	584,811
Net income	$ 109,214	$ 114,347	$ 111,023	$ 97,028	$ 103,161	$ 102,627	$ 97,226	$ 84,012	$ 431,612	$ 387,026	$ 360,830
Basic net income per share	$ 0.67	$ 0.70	$ 0.67	$ 0.59	$ 0.63	$ 0.62	$ 0.59	$ 0.51	$ 2.63	$ 2.35	$ 2.15
Diluted net income per share	$ 0.67	$ 0.70	$ 0.67	$ 0.59	$ 0.62	$ 0.62	$ 0.59	$ 0.50	$ 2.62	$ 2.33	$ 2.13
Basic weighted average shares outstanding	162,919	163,948	164,607	165,124	164,729	164,691	164,749	165,440	164,114	164,909	167,695
Dilutive effect of outstanding stock awards	906	523	587	640	1,346	885	1,016	1,135	627	1,063	1,499
Diluted weighted average shares outstanding	163,825	164,471	165,194	165,764	166,075	165,576	165,765	166,575	164,741	165,972	169,194
High	$ 76.76	$ 82.61	$ 81.53	$ 82.05	$ 81.02	$ 70.87	$ 62.15	$ 63.65
Low	$ 63.21	$ 62.30	$ 73.30	$ 70.32	$ 68.74	$ 54.50	$ 53.89	$ 51.16