Investment Securities - Mortgage-backed Asset Securities (Details) - USD ($) $ in Millions	Dec. 31, 2024	Dec. 31, 2023
Total mortgage and asset-backed securities and collateralized loan obligations
Investment securities
Fixed maturity investments, at fair value	$ 610.6	$ 598.5
Mortgage and asset-backed securities
Investment securities
Fixed maturity investments, at fair value	373.9	389.4
Total mortgage-backed securities
Investment securities
Fixed maturity investments, at fair value	370.0	366.4
Total mortgage-backed securities | Total non-agency
Investment securities
Fixed maturity investments, at fair value	0.4	0.2
Total mortgage-backed securities | Total agency
Investment securities
Fixed maturity investments, at fair value	369.6	366.2
Total mortgage-backed securities | FNMA
Investment securities
Fixed maturity investments, at fair value	198.3	198.8
Total mortgage-backed securities | FHLMC
Investment securities
Fixed maturity investments, at fair value	147.1	137.2
Total mortgage-backed securities | GNMA
Investment securities
Fixed maturity investments, at fair value	24.2	30.2
Total mortgage-backed securities | Commercial
Investment securities
Fixed maturity investments, at fair value	0.4	0.0
Total mortgage-backed securities | Residential
Investment securities
Fixed maturity investments, at fair value	0.0	0.2
Total other asset-backed securities
Investment securities
Fixed maturity investments, at fair value	3.9	23.0
Total other asset-backed securities | Vehicle receivables
Investment securities
Fixed maturity investments, at fair value	1.7	17.2
Total other asset-backed securities | Credit card receivables
Investment securities
Fixed maturity investments, at fair value	0.2	3.3
Total other asset-backed securities | Other
Investment securities
Fixed maturity investments, at fair value	2.0	2.5
Collateralized loan obligations
Investment securities
Fixed maturity investments, at fair value	236.7	209.1
Level 2 | Total mortgage and asset-backed securities and collateralized loan obligations
Investment securities
Fixed maturity investments, at fair value	610.6	598.5
Level 2 | Mortgage and asset-backed securities
Investment securities
Fixed maturity investments, at fair value	373.9	389.4
Level 2 | Total mortgage-backed securities
Investment securities
Fixed maturity investments, at fair value	370.0	366.4
Level 2 | Total mortgage-backed securities | Total non-agency
Investment securities
Fixed maturity investments, at fair value	0.4	0.2
Level 2 | Total mortgage-backed securities | Total agency
Investment securities
Fixed maturity investments, at fair value	369.6	366.2
Level 2 | Total mortgage-backed securities | FNMA
Investment securities
Fixed maturity investments, at fair value	198.3	198.8
Level 2 | Total mortgage-backed securities | FHLMC
Investment securities
Fixed maturity investments, at fair value	147.1	137.2
Level 2 | Total mortgage-backed securities | GNMA
Investment securities
Fixed maturity investments, at fair value	24.2	30.2
Level 2 | Total mortgage-backed securities | Commercial
Investment securities
Fixed maturity investments, at fair value	0.4	0.0
Level 2 | Total mortgage-backed securities | Residential
Investment securities
Fixed maturity investments, at fair value	0.0	0.2
Level 2 | Total other asset-backed securities
Investment securities
Fixed maturity investments, at fair value	3.9	23.0
Level 2 | Total other asset-backed securities | Vehicle receivables
Investment securities
Fixed maturity investments, at fair value	1.7	17.2
Level 2 | Total other asset-backed securities | Credit card receivables
Investment securities
Fixed maturity investments, at fair value	0.2	3.3
Level 2 | Total other asset-backed securities | Other
Investment securities
Fixed maturity investments, at fair value	2.0	2.5
Level 2 | Collateralized loan obligations
Investment securities
Fixed maturity investments, at fair value	236.7	209.1
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 | Commercial
Investment securities
Fixed maturity investments, at fair value	0.0	0.0
Level 3 | Total mortgage-backed securities | 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 | Vehicle receivables
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 | 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