SUPPLEMENTARY DATA (UNAUDITED) Summary of Unaudited Results of Operations for Each Quarter (Detail) - USD ($) $ / shares in Units, shares in Thousands, $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2015	Sep. 30, 2015	Jun. 30, 2015	Mar. 31, 2015	Dec. 31, 2014	Sep. 30, 2014	Jun. 30, 2014	Mar. 31, 2014	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Revenues:
Transportation	$ 2,867,301	$ 3,044,500	$ 3,130,722	$ 2,947,257	$ 3,014,251	$ 3,073,382	$ 3,042,102	$ 2,806,777	$ 11,989,780	$ 11,936,512	$ 11,082,942
Sourcing	343,552	374,753	414,366	353,633	342,951	393,980	460,816	335,808	1,486,304	1,533,555	1,669,134
Total revenues	3,210,853	3,419,253	3,545,088	3,300,890	3,357,202	3,467,362	3,502,918	3,142,585	13,476,084	13,470,067	12,752,076
Costs and expenses:
Purchased transportation and related services	2,323,376	2,484,409	2,582,374	2,452,112	2,536,440	2,575,619	2,555,959	2,376,388	9,842,271	10,044,406	9,373,797
Purchased products sourced for resale	316,700	346,269	378,696	323,668	318,946	364,179	425,922	308,962	1,365,333	1,418,009	1,542,184
Personnel expenses	268,190	264,077	263,999	255,144	235,117	244,621	238,986	220,297	1,051,410	939,021	826,661
Other selling, general, and administrative expenses	88,008	91,787	90,924	88,041	78,971	79,606	81,669	79,967	358,760	320,213	326,784
Total costs and expenses	2,996,274	3,186,542	3,315,993	3,118,965	3,169,474	3,264,025	3,302,536	2,985,614	12,617,774	12,721,649	12,069,426
Income from operations	214,579	232,711	229,095	181,925	187,728	203,337	200,382	156,971	858,310	748,418	682,650
Net income	$ 126,583	$ 139,432	$ 137,208	$ 106,476	$ 112,947	$ 124,981	$ 118,596	$ 93,187	$ 509,699	$ 449,711	$ 415,904
Basic net income per share (in dollars per share)	$ 0.88	$ 0.96	$ 0.94	$ 0.73	$ 0.77	$ 0.85	$ 0.80	$ 0.63	$ 3.52	$ 3.06	$ 2.65
Diluted net income per share (in dollars per share)	$ 0.88	$ 0.96	$ 0.94	$ 0.73	$ 0.77	$ 0.85	$ 0.80	$ 0.63	$ 3.51	$ 3.05	$ 2.65
Basic weighted average shares outstanding (in shares)	143,484	144,578	145,515	146,204	145,856	146,646	147,826	148,517	144,967	147,202	156,915
Dilutive effect of outstanding stock awards (in shares)	660	204	164	179	794	210	148	491	382	340	165
Diluted weighted average shares outstanding (in shares)	144,144	144,782	145,679	146,383	146,650	146,856	147,974	149,008	145,349	147,542	157,080
Market price range of common stock:
High (in dollars per share)	$ 73.34	$ 71.50	$ 73.09	$ 76.18	$ 77.49	$ 69.50	$ 64.09	$ 60.31
Low (in dollars per share)	$ 59.71	$ 61.64	$ 61.46	$ 67.11	$ 63.42	$ 63.09	$ 51.10	$ 50.21