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, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Revenues:
Transportation	$ 3,647,167	$ 3,433,701	$ 3,319,995	$ 3,102,043	$ 3,110,978	$ 2,998,583	$ 2,881,496	$ 2,713,688	$ 13,502,906	$ 11,704,745	$ 11,989,780
Sourcing	312,619	350,750	390,023	313,082	303,997	357,171	418,245	360,255	1,366,474	1,439,668	1,486,304
Total revenues	3,959,786	3,784,451	3,710,018	3,415,125	3,414,975	3,355,754	3,299,741	3,073,943	14,869,380	13,144,413	13,476,084
Costs and expenses:
Purchased transportation and related services	3,042,434	2,869,616	2,781,355	2,563,885	2,575,378	2,469,939	2,324,995	2,179,622	11,257,290	9,549,934	9,842,271
Purchased products sourced for resale	285,503	320,989	354,874	282,674	278,081	327,353	380,531	330,986	1,244,040	1,316,951	1,365,333
Personnel expenses	311,599	293,204	284,220	290,504	260,305	256,883	270,251	277,497	1,179,527	1,064,936	1,051,410
Other selling, general, and administrative expenses	109,374	106,177	107,749	90,104	107,646	90,312	90,217	86,886	413,404	375,061	358,760
Total costs and expenses	3,748,910	3,589,986	3,528,198	3,227,167	3,221,410	3,144,487	3,065,994	2,874,991	14,094,261	12,306,882	12,617,774
Income from operations	210,876	194,465	181,820	187,958	193,565	211,267	233,747	198,952	775,119	837,531	858,310
Net income	$ 152,556	$ 119,186	$ 111,071	$ 122,080	$ 122,303	$ 129,028	$ 143,090	$ 118,963	$ 504,893	$ 513,384	$ 509,699
Basic net income per share (in dollars per share)	$ 1.09	$ 0.85	$ 0.79	$ 0.86	$ 0.86	$ 0.90	$ 1.00	$ 0.83	$ 3.59	$ 3.60	$ 3.52
Diluted net income per share (in dollars per share)	$ 1.08	$ 0.85	$ 0.78	$ 0.86	$ 0.86	$ 0.90	$ 1.00	$ 0.83	$ 3.57	$ 3.59	$ 3.51
Basic weighted average shares outstanding (in shares)	139,572	140,422	141,061	141,484	141,711	142,611	142,998	143,525	140,610	142,706	144,967
Dilutive effect of outstanding stock awards (in shares)	1,152	600	526	374	453	272	218	133	772	285	382
Diluted weighted average shares outstanding (in shares)	140,724	141,022	141,587	141,858	142,164	142,883	143,216	143,658	141,382	142,991	145,349
Market price range of common stock:
High (in dollars per share)	$ 89.89	$ 76.16	$ 78.31	$ 81.16	$ 77.89	$ 75.69	$ 76.10	$ 75.11
Low (in dollars per share)	$ 74.30	$ 63.41	$ 66.33	$ 72.17	$ 65.57	$ 66.62	$ 69.84	$ 60.31
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Provision for income taxes									$ 223,570	$ 298,566	$ 313,082
Tax benefit for qualified production activity									19,700
Tax Cuts and Jobs Act of 2017, Provisional income tax benefit									12,100
Accounting Standards Update 2016-09
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Provision for income taxes									$ 13,700