REVENUE FROM CONTRACTS WITH CUSTOMERS - Impact on Consolidated Statements of Operations (Details) - USD ($) $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Revenues
Total revenues	$ 4,137,908	$ 4,291,900	$ 4,276,037	$ 3,925,327	$ 3,959,786	$ 3,784,451	$ 3,710,018	$ 3,415,125	$ 16,631,172	$ 14,869,380	$ 13,144,413
Costs and expenses
Personnel expenses	339,316	335,299	340,630	328,297	311,599	293,204	284,220	290,504	1,343,542	1,179,527	1,064,936
Other selling, general, and administrative expenses	118,950	112,772	111,845	106,043	109,374	106,177	107,749	90,104	449,610	413,404	375,061
Total costs and expenses	3,882,391	4,045,927	4,057,029	3,733,742	3,748,910	3,589,986	3,528,198	3,227,167	15,719,089	14,094,261	12,306,882
Income from operations	255,517	245,973	219,008	191,585	210,876	194,465	181,820	187,958	912,083	775,119	837,531
Interest and other expenses									(31,810)	(46,656)	(25,581)
Income before provision for income taxes									880,273	728,463	811,950
Provision for income taxes									215,768	223,570	298,566
Net income	187,150	175,895	159,163	142,297	152,556	119,186	111,071	122,080	664,505	504,893	513,384
ASU No. 2014-09
Costs and expenses
Net income	(4,200)	500	6,600	2,100
Calculated under Revenue Guidance in Effect before Topic 606
Revenues
Total revenues									16,698,041
Costs and expenses
Personnel expenses									1,343,159
Other selling, general, and administrative expenses									449,610
Total costs and expenses									15,792,866
Income from operations									905,175
Interest and other expenses									(31,810)
Income before provision for income taxes									873,365
Provision for income taxes									213,882
Net income									659,483
Difference between Revenue Guidance in Effect before and after Topic 606 | ASU No. 2014-09
Revenues
Total revenues									(66,869)
Costs and expenses
Personnel expenses									383
Other selling, general, and administrative expenses									0
Total costs and expenses									(73,777)
Income from operations									6,908
Interest and other expenses									0
Income before provision for income taxes									6,908
Provision for income taxes									1,886
Net income									5,022
Transportation and logistics services
Revenues
Total revenues	3,896,750	4,028,392	3,953,139	3,637,640	3,647,167	3,433,701	3,319,995	3,102,043	15,515,921	13,502,906	11,704,745
Costs and expenses
Purchased services and products	3,207,859	3,359,520	3,313,196	3,041,602	3,042,434	2,869,616	2,781,355	2,563,885	12,922,177	11,257,290	9,549,934
Transportation and logistics services | Calculated under Revenue Guidance in Effect before Topic 606
Revenues
Total revenues									15,462,328
Costs and expenses
Purchased services and products									12,875,875
Transportation and logistics services | Difference between Revenue Guidance in Effect before and after Topic 606 | ASU No. 2014-09
Revenues
Total revenues									53,593
Costs and expenses
Purchased services and products									46,302
Sourcing
Revenues
Total revenues	241,158	263,508	322,898	287,687	312,619	350,750	390,023	313,082	1,115,251	1,366,474	1,439,668
Costs and expenses
Purchased services and products	$ 216,266	$ 238,336	$ 291,358	$ 257,800	$ 285,503	$ 320,989	$ 354,874	$ 282,674	1,003,760	$ 1,244,040	$ 1,316,951
Sourcing | Calculated under Revenue Guidance in Effect before Topic 606
Revenues
Total revenues									1,235,713
Costs and expenses
Purchased services and products									1,124,222
Sourcing | Difference between Revenue Guidance in Effect before and after Topic 606 | ASU No. 2014-09
Revenues
Total revenues									(120,462)
Costs and expenses
Purchased services and products									$ (120,462)