REVENUE - Disaggregation of Revenue (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2021	Sep. 30, 2020	Sep. 30, 2021	Sep. 30, 2020	Dec. 31, 2020	Dec. 31, 2019
Revenue, Major Customer [Line Items]
Total operating revenues	$ 1,750.0	$ 1,497.6	$ 5,131.1	$ 4,515.5
Deferred carried interest liabilities	60.5		60.5		$ 58.0	$ 45.8
Performance revenue recognized	0.0	0.0	0.0	0.0
Bermuda
Revenue, Major Customer [Line Items]
Total operating revenues	0.0	0.0	0.0	0.0
Americas
Revenue, Major Customer [Line Items]
Total operating revenues	1,329.8	1,118.2	3,842.8	3,356.2
UK
Revenue, Major Customer [Line Items]
Total operating revenues	157.7	148.6	486.0	465.3
EMEA ex UK (Europe, Middle East and Africa)
Revenue, Major Customer [Line Items]
Total operating revenues	181.0	158.2	541.5	468.3
Asia Pacific
Revenue, Major Customer [Line Items]
Total operating revenues	$ 81.5	$ 72.6	$ 260.8	$ 225.7