Investment Securities - Mortgage-backed Asset Securities (Details) - USD ($) $ in Millions	Sep. 30, 2025	Dec. 31, 2024
Total mortgage and asset-backed securities and collateralized loan obligations
Debt and Equity Securities, FV-NI [Line Items]
Fixed maturity investments, at fair value	$ 759.4	$ 610.6
Total mortgage and asset-backed securities
Debt and Equity Securities, FV-NI [Line Items]
Fixed maturity investments, at fair value	397.3	373.9
Mortgage-backed securities:
Debt and Equity Securities, FV-NI [Line Items]
Fixed maturity investments, at fair value	364.2	370.0
Other asset-backed securities:
Debt and Equity Securities, FV-NI [Line Items]
Fixed maturity investments, at fair value	33.1	3.9
Collateralized loan obligations
Debt and Equity Securities, FV-NI [Line Items]
Fixed maturity investments, at fair value	362.1	236.7
Level 2 | Total mortgage and asset-backed securities and collateralized loan obligations
Debt and Equity Securities, FV-NI [Line Items]
Fixed maturity investments, at fair value	759.4	610.6
Level 2 | Total mortgage and asset-backed securities
Debt and Equity Securities, FV-NI [Line Items]
Fixed maturity investments, at fair value	397.3	373.9
Level 2 | Mortgage-backed securities:
Debt and Equity Securities, FV-NI [Line Items]
Fixed maturity investments, at fair value	364.2	370.0
Level 2 | Other asset-backed securities:
Debt and Equity Securities, FV-NI [Line Items]
Fixed maturity investments, at fair value	33.1	3.9
Level 2 | Collateralized loan obligations
Debt and Equity Securities, FV-NI [Line Items]
Fixed maturity investments, at fair value	362.1	236.7
Level 3 | Total mortgage and asset-backed securities and collateralized loan obligations
Debt and Equity Securities, FV-NI [Line Items]
Fixed maturity investments, at fair value	0.0	0.0
Level 3 | Total mortgage and asset-backed securities
Debt and Equity Securities, FV-NI [Line Items]
Fixed maturity investments, at fair value	0.0	0.0
Level 3 | Mortgage-backed securities:
Debt and Equity Securities, FV-NI [Line Items]
Fixed maturity investments, at fair value	0.0	0.0
Level 3 | Other asset-backed securities:
Debt and Equity Securities, FV-NI [Line Items]
Fixed maturity investments, at fair value	0.0	0.0
Level 3 | Collateralized loan obligations
Debt and Equity Securities, FV-NI [Line Items]
Fixed maturity investments, at fair value	0.0	0.0
Total non-agency | Mortgage-backed securities:
Debt and Equity Securities, FV-NI [Line Items]
Fixed maturity investments, at fair value	0.4	0.4
Total non-agency | Level 2 | Mortgage-backed securities:
Debt and Equity Securities, FV-NI [Line Items]
Fixed maturity investments, at fair value	0.4	0.4
Total non-agency | Level 3 | Mortgage-backed securities:
Debt and Equity Securities, FV-NI [Line Items]
Fixed maturity investments, at fair value	0.0	0.0
Total agency | Mortgage-backed securities:
Debt and Equity Securities, FV-NI [Line Items]
Fixed maturity investments, at fair value	363.8	369.6
Total agency | Level 2 | Mortgage-backed securities:
Debt and Equity Securities, FV-NI [Line Items]
Fixed maturity investments, at fair value	363.8	369.6
Total agency | Level 3 | Mortgage-backed securities:
Debt and Equity Securities, FV-NI [Line Items]
Fixed maturity investments, at fair value	0.0	0.0
FNMA | Mortgage-backed securities:
Debt and Equity Securities, FV-NI [Line Items]
Fixed maturity investments, at fair value	197.9	198.3
FNMA | Level 2 | Mortgage-backed securities:
Debt and Equity Securities, FV-NI [Line Items]
Fixed maturity investments, at fair value	197.9	198.3
FNMA | Level 3 | Mortgage-backed securities:
Debt and Equity Securities, FV-NI [Line Items]
Fixed maturity investments, at fair value	0.0	0.0
FHLMC | Mortgage-backed securities:
Debt and Equity Securities, FV-NI [Line Items]
Fixed maturity investments, at fair value	143.1	147.1
FHLMC | Level 2 | Mortgage-backed securities:
Debt and Equity Securities, FV-NI [Line Items]
Fixed maturity investments, at fair value	143.1	147.1
FHLMC | Level 3 | Mortgage-backed securities:
Debt and Equity Securities, FV-NI [Line Items]
Fixed maturity investments, at fair value	0.0	0.0
GNMA | Mortgage-backed securities:
Debt and Equity Securities, FV-NI [Line Items]
Fixed maturity investments, at fair value	22.8	24.2
GNMA | Level 2 | Mortgage-backed securities:
Debt and Equity Securities, FV-NI [Line Items]
Fixed maturity investments, at fair value	22.8	24.2
GNMA | Level 3 | Mortgage-backed securities:
Debt and Equity Securities, FV-NI [Line Items]
Fixed maturity investments, at fair value	0.0	0.0
Non-agency: Commercial | Mortgage-backed securities:
Debt and Equity Securities, FV-NI [Line Items]
Fixed maturity investments, at fair value	0.4	0.4
Non-agency: Commercial | Level 2 | Mortgage-backed securities:
Debt and Equity Securities, FV-NI [Line Items]
Fixed maturity investments, at fair value	0.4	0.4
Non-agency: Commercial | Level 3 | Mortgage-backed securities:
Debt and Equity Securities, FV-NI [Line Items]
Fixed maturity investments, at fair value	0.0	0.0
Vehicle receivables | Other asset-backed securities:
Debt and Equity Securities, FV-NI [Line Items]
Fixed maturity investments, at fair value	17.9	1.7
Vehicle receivables | Level 2 | Other asset-backed securities:
Debt and Equity Securities, FV-NI [Line Items]
Fixed maturity investments, at fair value	17.9	1.7
Vehicle receivables | Level 3 | Other asset-backed securities:
Debt and Equity Securities, FV-NI [Line Items]
Fixed maturity investments, at fair value	0.0	0.0
Credit card receivables | Other asset-backed securities:
Debt and Equity Securities, FV-NI [Line Items]
Fixed maturity investments, at fair value	10.7	0.2
Credit card receivables | Level 2 | Other asset-backed securities:
Debt and Equity Securities, FV-NI [Line Items]
Fixed maturity investments, at fair value	10.7	0.2
Credit card receivables | Level 3 | Other asset-backed securities:
Debt and Equity Securities, FV-NI [Line Items]
Fixed maturity investments, at fair value	0.0	0.0
Other | Other asset-backed securities:
Debt and Equity Securities, FV-NI [Line Items]
Fixed maturity investments, at fair value	4.5	2.0
Other | Level 2 | Other asset-backed securities:
Debt and Equity Securities, FV-NI [Line Items]
Fixed maturity investments, at fair value	4.5	2.0
Other | Level 3 | Other asset-backed securities:
Debt and Equity Securities, FV-NI [Line Items]
Fixed maturity investments, at fair value	$ 0.0	$ 0.0