SUPPLEMENTARY DATA (UNAUDITED) - Summary of Unaudited Results of Operations for Each Quarter (Details) - USD ($) $ / shares in Units, shares in Thousands, $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2015	Sep. 30, 2015	Jun. 30, 2015	Mar. 31, 2015	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Revenues:
Transportation	$ 3,110,978	$ 2,998,583	$ 2,881,496	$ 2,713,688	$ 2,867,301	$ 3,044,500	$ 3,130,722	$ 2,947,257	$ 11,704,745	$ 11,989,780	$ 11,936,512
Sourcing	303,997	357,171	418,245	360,255	343,552	374,753	414,366	353,633	1,439,668	1,486,304	1,533,555
Total revenues	3,414,975	3,355,754	3,299,741	3,073,943	3,210,853	3,419,253	3,545,088	3,300,890	13,144,413	13,476,084	13,470,067
Costs and expenses:
Purchased transportation and related services	2,575,378	2,469,939	2,324,995	2,179,622	2,323,376	2,484,409	2,582,374	2,452,112	9,549,934	9,842,271	10,044,406
Purchased products sourced for resale	278,081	327,353	380,531	330,986	316,700	346,269	378,696	323,668	1,316,951	1,365,333	1,418,009
Personnel expenses	260,305	256,883	270,251	277,497	268,190	264,077	263,999	255,144	1,064,936	1,051,410	939,021
Other selling, general, and administrative expenses	107,646	90,312	90,217	86,886	88,008	91,787	90,924	88,041	375,061	358,760	320,213
Total costs and expenses	3,221,410	3,144,487	3,065,994	2,874,991	2,996,274	3,186,542	3,315,993	3,118,965	12,306,882	12,617,774	12,721,649
Income from operations	193,565	211,267	233,747	198,952	214,579	232,711	229,095	181,925	837,531	858,310	748,418
Net income	$ 122,303	$ 129,028	$ 143,090	$ 118,963	$ 126,583	$ 139,432	$ 137,208	$ 106,476	$ 513,384	$ 509,699	$ 449,711
Basic net income per share (in dollars per share)	$ 0.86	$ 0.90	$ 1.00	$ 0.83	$ 0.88	$ 0.96	$ 0.94	$ 0.73	$ 3.60	$ 3.52	$ 3.06
Diluted net income per share (in dollars per share)	$ 0.86	$ 0.90	$ 1.00	$ 0.83	$ 0.88	$ 0.96	$ 0.94	$ 0.73	$ 3.59	$ 3.51	$ 3.05
Basic weighted average shares outstanding (in shares)	141,711	142,611	142,998	143,525	143,484	144,578	145,515	146,204	142,706	144,967	147,202
Dilutive effect of outstanding stock awards (in shares)	453	272	218	133	660	204	164	179	285	382	340
Diluted weighted average shares outstanding (in shares)	142,164	142,883	143,216	143,658	144,144	144,782	145,679	146,383	142,991	145,349	147,542
Market price range of common stock:
High (in dollars per share)	$ 77.89	$ 75.69	$ 76.10	$ 75.11	$ 73.34	$ 71.50	$ 73.09	$ 76.18
Low (in dollars per share)	$ 65.57	$ 66.62	$ 69.84	$ 60.31	$ 59.71	$ 61.64	$ 61.46	$ 67.11