GOODWILL AND OTHER INTANGIBLE ASSETS - Carrying Amount of Goodwill (Details) $ in Thousands	9 Months Ended
	Sep. 30, 2021 USD ($)
Goodwill [Roll Forward]
Balance, beginning of period	$ 1,487,187
Acquisitions	10,997
Foreign currency translation	(11,985)
Balance, end of period	1,486,199
NAST
Goodwill [Roll Forward]
Balance, beginning of period	1,203,972
Acquisitions	243
Foreign currency translation	(7,894)
Balance, end of period	1,196,321
Global Forwarding
Goodwill [Roll Forward]
Balance, beginning of period	213,982
Acquisitions	0
Foreign currency translation	(2,668)
Balance, end of period	211,314
All Other and Corporate
Goodwill [Roll Forward]
Balance, beginning of period	69,233
Acquisitions	10,754
Foreign currency translation	(1,423)
Balance, end of period	$ 78,564