GOODWILL AND INTANGIBLE ASSETS - SCHEDULE OF GOODWILL (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019
Goodwill [Roll Forward]
Beginning balance January 1	$ 1,759,040	$ 1,722,603
Ending balance December 31	3,187,169	1,759,040
Goodwill, Acquired During Period	1,384,027	5,767
Goodwill, Foreign Currency Translation Gain (Loss)	44,102	30,670
Disposal Group, Including Discontinued Operation, Goodwill, Current	26,100
EES
Goodwill [Roll Forward]
Beginning balance January 1	573,447	542,704
Ending balance December 31	853,456	573,447
Goodwill, Acquired During Period	264,538	5,767
Goodwill, Foreign Currency Translation Gain (Loss)	15,471	24,976
Goodwill, Period Increase (Decrease)	20,200
UBS
Goodwill [Roll Forward]
Beginning balance January 1	949,882	945,450
Ending balance December 31	1,218,213	949,882
Goodwill, Acquired During Period	250,553	0
Goodwill, Foreign Currency Translation Gain (Loss)	17,778	4,432
Goodwill, Period Increase (Decrease)	11,700
CSS
Goodwill [Roll Forward]
Beginning balance January 1	235,711	234,449
Ending balance December 31	1,115,500	235,711
Goodwill, Acquired During Period	868,936	0
Goodwill, Foreign Currency Translation Gain (Loss)	10,853	$ 1,262
Goodwill, Period Increase (Decrease)	$ 2,000