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Investment Securities - Mortgage and Asset-Backed Securities and Collateralized Loan Obligations (Details) - USD ($) $ in Millions	Sep. 30, 2023	Dec. 31, 2022
Total mortgage and asset-backed securities and collateralized loan obligations
Debt and Equity Securities, FV-NI [Line Items]
Fixed maturity investments, at fair value	$ 517.0	$ 437.1
Total mortgage and asset-backed securities
Debt and Equity Securities, FV-NI [Line Items]
Fixed maturity investments, at fair value	325.4	254.2
Mortgage-backed securities:
Debt and Equity Securities, FV-NI [Line Items]
Fixed maturity investments, at fair value	306.2	231.9
Other asset-backed securities:
Debt and Equity Securities, FV-NI [Line Items]
Fixed maturity investments, at fair value	19.2	22.3
Collateralized loan obligations
Debt and Equity Securities, FV-NI [Line Items]
Fixed maturity investments, at fair value	191.6	182.9
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	517.0	437.1
Level 2 | Total mortgage and asset-backed securities
Debt and Equity Securities, FV-NI [Line Items]
Fixed maturity investments, at fair value	325.4	254.2
Level 2 | Mortgage-backed securities:
Debt and Equity Securities, FV-NI [Line Items]
Fixed maturity investments, at fair value	306.2	231.9
Level 2 | Other asset-backed securities:
Debt and Equity Securities, FV-NI [Line Items]
Fixed maturity investments, at fair value	19.2	22.3
Level 2 | Collateralized loan obligations
Debt and Equity Securities, FV-NI [Line Items]
Fixed maturity investments, at fair value	191.6	182.9
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	0.2	0.3
Total non-agency | Level 2 | Mortgage-backed securities:
Debt and Equity Securities, FV-NI [Line Items]
Fixed maturity investments, at fair value	0.2	0.3
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	306.0	231.6
Total agency | Level 2 | Mortgage-backed securities:
Debt and Equity Securities, FV-NI [Line Items]
Fixed maturity investments, at fair value	306.0	231.6
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	162.5	124.5
FNMA | Level 2 | Mortgage-backed securities:
Debt and Equity Securities, FV-NI [Line Items]
Fixed maturity investments, at fair value	162.5	124.5
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	114.3	78.8
FHLMC | Level 2 | Mortgage-backed securities:
Debt and Equity Securities, FV-NI [Line Items]
Fixed maturity investments, at fair value	114.3	78.8
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	29.2	28.3
GNMA | Level 2 | Mortgage-backed securities:
Debt and Equity Securities, FV-NI [Line Items]
Fixed maturity investments, at fair value	29.2	28.3
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: Residential | Mortgage-backed securities:
Debt and Equity Securities, FV-NI [Line Items]
Fixed maturity investments, at fair value	0.2	0.3
Non-agency: Residential | Level 2 | Mortgage-backed securities:
Debt and Equity Securities, FV-NI [Line Items]
Fixed maturity investments, at fair value	0.2	0.3
Non-agency: Residential | Level 3 | Mortgage-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	3.2	11.9
Credit card receivables | Level 2 | Other asset-backed securities:
Debt and Equity Securities, FV-NI [Line Items]
Fixed maturity investments, at fair value	3.2	11.9
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
Vehicle receivables | Other asset-backed securities:
Debt and Equity Securities, FV-NI [Line Items]
Fixed maturity investments, at fair value	13.5	10.4
Vehicle receivables | Level 2 | Other asset-backed securities:
Debt and Equity Securities, FV-NI [Line Items]
Fixed maturity investments, at fair value	13.5	10.4
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
Other | Other asset-backed securities:
Debt and Equity Securities, FV-NI [Line Items]
Fixed maturity investments, at fair value	2.5	0.0
Other | Level 2 | Other asset-backed securities:
Debt and Equity Securities, FV-NI [Line Items]
Fixed maturity investments, at fair value	2.5	0.0
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

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Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef
-Topic 320
-SubTopic 10
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Reference 2: http://www.xbrl.org/2003/role/disclosureRef
-Topic 210
-SubTopic 10
-Name Accounting Standards Codification
-Section S99
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