Investment Securities - Debt Securities Issued by Corporation (Details) - Debt securities issued by corporations - USD ($) $ in Millions	Mar. 31, 2025	Dec. 31, 2024
Debt Securities, Trading, Credit Quality Indicator [Line Items]
Fixed maturity investments, at fair value	$ 1,398.5	$ 1,414.2
AAA
Debt Securities, Trading, Credit Quality Indicator [Line Items]
Fixed maturity investments, at fair value	11.9	14.6
AA
Debt Securities, Trading, Credit Quality Indicator [Line Items]
Fixed maturity investments, at fair value	89.5	91.2
A
Debt Securities, Trading, Credit Quality Indicator [Line Items]
Fixed maturity investments, at fair value	623.7	607.2
BBB
Debt Securities, Trading, Credit Quality Indicator [Line Items]
Fixed maturity investments, at fair value	659.1	688.7
BB
Debt Securities, Trading, Credit Quality Indicator [Line Items]
Fixed maturity investments, at fair value	6.8	5.4
Other
Debt Securities, Trading, Credit Quality Indicator [Line Items]
Fixed maturity investments, at fair value	$ 7.5	$ 7.1

Amount of investment in debt security measured at fair value with change in fair value recognized in net income (trading).

Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef
-Topic 820
-SubTopic 10
-Name Accounting Standards Codification
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-Publisher FASB
-URI https://asc.fasb.org/1943274/2147482106/820-10-50-2

Reference 2: http://www.xbrl.org/2009/role/commonPracticeRef
-Topic 820
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Reference 3: http://www.xbrl.org/2009/role/commonPracticeRef
-Topic 820
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Reference 4: http://www.xbrl.org/2003/role/disclosureRef
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Reference 5: http://www.xbrl.org/2009/role/commonPracticeRef
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Reference 6: http://www.xbrl.org/2003/role/disclosureRef
-Topic 210
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-Publisher FASB
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Reference 7: http://www.xbrl.org/2003/role/exampleRef
-Topic 210
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-URI https://asc.fasb.org/1943274/2147483467/210-10-45-1

Name:	us-gaap_TradingSecuritiesDebt
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant