DERIVATIVE INSTRUMENTS - Narrative (Details) - Derivative instruments not designated as hedges - USD ($) $ in Millions	Mar. 31, 2023	Dec. 31, 2022
Interest rate futures | Long
Derivative [Line Items]
Notional amount	$ 2,700.0	$ 2,400.0
Interest rate futures | Short
Derivative [Line Items]
Notional amount	1,000.0	500.0
Foreign currency forward contracts, underwriting and non-investment operations | Long
Derivative [Line Items]
Notional amount	821.0	861.7
Foreign currency forward contracts, underwriting and non-investment operations | Short
Derivative [Line Items]
Notional amount	262.1	172.4
Foreign currency forward contracts, investment operations | Long
Derivative [Line Items]
Notional amount	416.3	225.6
Foreign currency forward contracts, investment operations | Short
Derivative [Line Items]
Notional amount	161.3	86.3
Credit default swaps | Long
Derivative [Line Items]
Notional amount	13.4	953.4
Credit default swaps | Short
Derivative [Line Items]
Notional amount	1,300.0	13.1
Equity futures | Long
Derivative [Line Items]
Notional amount		$ 116.0
Equity futures | Short
Derivative [Line Items]
Notional amount	$ 108.2