SEGMENT REPORTING - Reportable Segment Information (Details) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2019 USD ($) employee	Sep. 30, 2018 USD ($) employee	Sep. 30, 2019 USD ($) employee	Sep. 30, 2018 USD ($) employee	Dec. 31, 2018 USD ($)
Segment Reporting Information [Line Items]
Total revenues	$ 3,856,132	$ 4,291,900	$ 11,516,182	$ 12,493,264
Net revenues	633,431	694,044	2,007,447	1,991,452
Income (loss) from operations	201,085	245,973	653,170	656,566
Depreciation and amortization	25,480	23,923	75,122	72,402
Total assets	$ 4,636,549	$ 4,492,722	$ 4,636,549	$ 4,492,722	$ 4,427,412
Average headcount (employee) | employee	15,782	15,291	15,582	15,189
Operating Segments | NAST
Segment Reporting Information [Line Items]
Total revenues	$ 2,826,308	$ 3,224,906	$ 8,495,145	$ 9,296,510
Net revenues	433,760	499,463	1,406,728	1,397,571
Income (loss) from operations	176,200	223,893	592,215	591,774
Depreciation and amortization	5,734	6,286	18,124	18,905
Total assets	$ 2,649,259	$ 2,739,569	$ 2,649,259	$ 2,739,569
Average headcount (employee) | employee	7,448	7,454	7,436	7,375
Operating Segments | Global Forwarding
Segment Reporting Information [Line Items]
Total revenues	$ 597,695	$ 639,268	$ 1,727,745	$ 1,810,619
Net revenues	135,815	134,101	404,987	401,169
Income (loss) from operations	24,676	23,835	65,497	61,844
Depreciation and amortization	9,186	8,735	27,427	26,397
Total assets	$ 995,137	$ 944,928	$ 995,137	$ 944,928
Average headcount (employee) | employee	4,790	4,684	4,748	4,725
Operating Segments | All Other and Corporate
Segment Reporting Information [Line Items]
Total revenues	$ 432,129	$ 427,726	$ 1,293,292	$ 1,386,135
Net revenues	63,856	60,480	195,732	192,712
Income (loss) from operations	209	(1,755)	(4,542)	2,948
Depreciation and amortization	10,560	8,902	29,571	27,100
Total assets	$ 992,153	$ 808,225	$ 992,153	$ 808,225
Average headcount (employee) | employee	3,544	3,153	3,398	3,089