INVESTMENTS - SCHEDULE OF STRUCTURED SECURITIES (Details) $ in Millions	Dec. 31, 2022 USD ($)
Structured Securities [Line Items]
Amortized cost	$ 6,630.2
Estimated fair value, Amount	$ 6,018.8
Estimated fair value, Percent of fixed maturities	29.60%
Asset-backed securities
Structured Securities [Line Items]
Amortized cost	$ 1,435.7
Estimated fair value, Amount	$ 1,287.0
Estimated fair value, Percent of fixed maturities	6.30%
Agency residential mortgage-backed securities
Structured Securities [Line Items]
Amortized cost	$ 174.3
Estimated fair value, Amount	$ 175.0
Estimated fair value, Percent of fixed maturities	0.90%
Non-agency residential mortgage-backed securities
Structured Securities [Line Items]
Amortized cost	$ 1,700.4
Estimated fair value, Amount	$ 1,548.5
Estimated fair value, Percent of fixed maturities	7.60%
Collateralized loan obligations
Structured Securities [Line Items]
Amortized cost	$ 825.2
Estimated fair value, Amount	$ 785.9
Estimated fair value, Percent of fixed maturities	3.90%
Commercial mortgage-backed securities
Structured Securities [Line Items]
Amortized cost	$ 2,494.6
Estimated fair value, Amount	$ 2,222.4
Estimated fair value, Percent of fixed maturities	10.90%