Investment Securities - Mortgage-backed Asset Securities (Details) - USD ($) $ in Millions	Dec. 31, 2023	Dec. 31, 2022
Total mortgage and asset-backed securities and collateralized loan obligations
Investment securities
Fixed maturity investments, at fair value	$ 598.5	$ 437.1
Mortgage and asset-backed securities
Investment securities
Fixed maturity investments, at fair value	389.4	254.2
Total mortgage-backed securities
Investment securities
Fixed maturity investments, at fair value	366.4	231.9
Total mortgage-backed securities | Total non-agency
Investment securities
Fixed maturity investments, at fair value	0.2	0.3
Total mortgage-backed securities | Total agency
Investment securities
Fixed maturity investments, at fair value	366.2	231.6
Total mortgage-backed securities | FNMA
Investment securities
Fixed maturity investments, at fair value	198.8	124.5
Total mortgage-backed securities | FHLMC
Investment securities
Fixed maturity investments, at fair value	137.2	78.8
Total mortgage-backed securities | GNMA
Investment securities
Fixed maturity investments, at fair value	30.2	28.3
Total mortgage-backed securities | Non-agency: Residential
Investment securities
Fixed maturity investments, at fair value	0.2	0.3
Total other asset-backed securities
Investment securities
Fixed maturity investments, at fair value	23.0	22.3
Total other asset-backed securities | Credit card receivables
Investment securities
Fixed maturity investments, at fair value	3.3	11.9
Total other asset-backed securities | Vehicle receivables
Investment securities
Fixed maturity investments, at fair value	17.2	10.4
Total other asset-backed securities | Other
Investment securities
Fixed maturity investments, at fair value	2.5	0.0
Collateralized loan obligations
Investment securities
Fixed maturity investments, at fair value	209.1	182.9
Level 2 | Total mortgage and asset-backed securities and collateralized loan obligations
Investment securities
Fixed maturity investments, at fair value	598.5	437.1
Level 2 | Mortgage and asset-backed securities
Investment securities
Fixed maturity investments, at fair value	389.4	254.2
Level 2 | Total mortgage-backed securities
Investment securities
Fixed maturity investments, at fair value	366.4	231.9
Level 2 | Total mortgage-backed securities | Total non-agency
Investment securities
Fixed maturity investments, at fair value	0.2	0.3
Level 2 | Total mortgage-backed securities | Total agency
Investment securities
Fixed maturity investments, at fair value	366.2	231.6
Level 2 | Total mortgage-backed securities | FNMA
Investment securities
Fixed maturity investments, at fair value	198.8	124.5
Level 2 | Total mortgage-backed securities | FHLMC
Investment securities
Fixed maturity investments, at fair value	137.2	78.8
Level 2 | Total mortgage-backed securities | GNMA
Investment securities
Fixed maturity investments, at fair value	30.2	28.3
Level 2 | Total mortgage-backed securities | Non-agency: Residential
Investment securities
Fixed maturity investments, at fair value	0.2	0.3
Level 2 | Total other asset-backed securities
Investment securities
Fixed maturity investments, at fair value	23.0	22.3
Level 2 | Total other asset-backed securities | Credit card receivables
Investment securities
Fixed maturity investments, at fair value	3.3	11.9
Level 2 | Total other asset-backed securities | Vehicle receivables
Investment securities
Fixed maturity investments, at fair value	17.2	10.4
Level 2 | Total other asset-backed securities | Other
Investment securities
Fixed maturity investments, at fair value	2.5	0.0
Level 2 | Collateralized loan obligations
Investment securities
Fixed maturity investments, at fair value	209.1	182.9
Level 3 | Total mortgage and asset-backed securities and collateralized loan obligations
Investment securities
Fixed maturity investments, at fair value	0.0	0.0
Level 3 | Mortgage and asset-backed securities
Investment securities
Fixed maturity investments, at fair value	0.0	0.0
Level 3 | Total mortgage-backed securities
Investment securities
Fixed maturity investments, at fair value	0.0	0.0
Level 3 | Total mortgage-backed securities | Total non-agency
Investment securities
Fixed maturity investments, at fair value	0.0	0.0
Level 3 | Total mortgage-backed securities | Total agency
Investment securities
Fixed maturity investments, at fair value	0.0	0.0
Level 3 | Total mortgage-backed securities | FNMA
Investment securities
Fixed maturity investments, at fair value	0.0	0.0
Level 3 | Total mortgage-backed securities | FHLMC
Investment securities
Fixed maturity investments, at fair value	0.0	0.0
Level 3 | Total mortgage-backed securities | GNMA
Investment securities
Fixed maturity investments, at fair value	0.0	0.0
Level 3 | Total mortgage-backed securities | Non-agency: Residential
Investment securities
Fixed maturity investments, at fair value	0.0	0.0
Level 3 | Total other asset-backed securities
Investment securities
Fixed maturity investments, at fair value	0.0	0.0
Level 3 | Total other asset-backed securities | Credit card receivables
Investment securities
Fixed maturity investments, at fair value	0.0	0.0
Level 3 | Total other asset-backed securities | Vehicle receivables
Investment securities
Fixed maturity investments, at fair value	0.0	0.0
Level 3 | Total other asset-backed securities | Other
Investment securities
Fixed maturity investments, at fair value	0.0	0.0
Level 3 | Collateralized loan obligations
Investment securities
Fixed maturity investments, at fair value	$ 0.0	$ 0.0