Investment Securities - Debt Securities Issued by Corporation (Details) - Debt securities issued by corporations - USD ($) $ in Millions	Sep. 30, 2024	Dec. 31, 2023
Debt Securities, Trading, Credit Quality Indicator [Line Items]
Fixed maturity investments, at fair value	$ 1,310.9	$ 1,045.0
AAA
Debt Securities, Trading, Credit Quality Indicator [Line Items]
Fixed maturity investments, at fair value	15.0	11.5
AA
Debt Securities, Trading, Credit Quality Indicator [Line Items]
Fixed maturity investments, at fair value	82.3	83.8
A
Debt Securities, Trading, Credit Quality Indicator [Line Items]
Fixed maturity investments, at fair value	577.6	552.4
BBB
Debt Securities, Trading, Credit Quality Indicator [Line Items]
Fixed maturity investments, at fair value	625.5	390.9
BB
Debt Securities, Trading, Credit Quality Indicator [Line Items]
Fixed maturity investments, at fair value	3.2	0.0
Other
Debt Securities, Trading, Credit Quality Indicator [Line Items]
Fixed maturity investments, at fair value	$ 7.3	$ 6.4

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
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Reference 2: http://www.xbrl.org/2009/role/commonPracticeRef
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Reference 3: http://www.xbrl.org/2009/role/commonPracticeRef
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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
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Reference 7: http://www.xbrl.org/2003/role/exampleRef
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Name:	us-gaap_TradingSecuritiesDebt
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant