SUPPLEMENTARY DATA (UNAUDITED) 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, 2014	Sep. 30, 2014	Jun. 30, 2014	Mar. 31, 2014	Dec. 31, 2013	Sep. 30, 2013	Jun. 30, 2013	Mar. 31, 2013	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Revenues:
Transportation	$ 3,010,291	$ 3,069,056	$ 3,038,923	$ 2,803,704	$ 2,767,550	$ 2,880,901	$ 2,818,077	$ 2,603,182	$ 11,921,974	$ 11,069,710	$ 9,685,415
Sourcing	342,951	393,980	460,816	335,808	382,098	432,373	466,811	387,852	1,533,555	1,669,134	1,620,183
Payment Services	3,960	4,326	3,179	3,073	3,234	3,391	3,374	3,233	14,538	13,232	53,515
Total revenues	3,357,202	3,467,362	3,502,918	3,142,585	3,152,882	3,316,665	3,288,262	2,994,267	13,470,067	12,752,076	11,359,113
Costs and expenses:
Purchased transportation and related services	2,535,985	2,575,069	2,555,371	2,375,825	2,351,530	2,450,923	2,386,932	2,181,930	10,042,250	9,371,315	8,157,278
Purchased products sourced for resale	318,946	364,179	425,922	308,962	356,299	401,820	428,059	356,006	1,418,009	1,542,184	1,483,745
Purchased payment services	455	550	588	563	588	616	669	609	2,156	2,482	519
Personnel expenses	235,117	244,621	238,986	220,297	203,619	204,388	206,009	212,645	939,021	826,661	766,006
Other selling, general, and administrative expenses	78,971	79,606	81,669	79,967	85,733	82,563	84,117	74,371	320,213	326,784	276,245
Total costs and expenses	3,169,474	3,264,025	3,302,536	2,985,614	2,997,769	3,140,310	3,105,786	2,825,561	12,721,649	12,069,426	10,683,793
Income from operations	187,728	203,337	200,382	156,971	155,113	176,355	182,476	168,706	748,418	682,650	675,320
Net income	$ 112,947	$ 124,981	$ 118,596	$ 93,187	$ 92,952	$ 107,737	$ 111,872	$ 103,343	$ 449,711	$ 415,904	$ 593,804
Basic net income per share (in dollars per share)	$ 0.77	$ 0.85	$ 0.80	$ 0.63	$ 0.62	$ 0.69	$ 0.70	$ 0.64	$ 3.06	$ 2.65	$ 3.68
Diluted net income per share (in dollars per share)	$ 0.77	$ 0.85	$ 0.80	$ 0.63	$ 0.62	$ 0.69	$ 0.70	$ 0.64	$ 3.05	$ 2.65	$ 3.67
Basic weighted average shares outstanding (in shares)	145,856	146,646	147,826	148,517	150,856	156,924	159,818	160,637	147,202	156,915	161,557
Dilutive effect of outstanding stock awards (in shares)	794	210	148	491	274	120	99	53	340	165	389
Diluted weighted average shares outstanding (in shares)	146,650	146,856	147,974	149,008	151,130	157,044	159,917	160,690	147,542	157,080	161,946
Market price range of common stock:
High (in dollars per share)	$ 77.49	$ 69.50	$ 64.09	$ 60.31	$ 61.94	$ 62.46	$ 61.91	$ 67.93
Low (in dollars per share)	$ 63.42	$ 63.09	$ 51.10	$ 50.21	$ 55.92	$ 55.26	$ 53.74	$ 55.81