Fair Value - Summary of Changes in Fair Value Associated with Level 3 Assets (Details) - USD ($) $ in Millions	3 Months Ended
Mar. 31, 2024	Mar. 31, 2023
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Fair value, beginning balance	$ 110.1	$ 121.9
Calls/ Maturities/ Paydowns	3.7	0.5
Purchases	0.0	0.0
Sales	0.0	0.0
Net Realized (Gain)/Loss on Sales	0.0	0.0
Change in Valuation	0.0	0.0
Net Transfers In (Out)	0.0	0.0
Fair value, ending balance	113.8	122.4
Corporate debt securities
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Fair value, beginning balance	3.0
Calls/ Maturities/ Paydowns	0.0
Purchases	0.0
Sales	0.0
Net Realized (Gain)/Loss on Sales	0.0
Change in Valuation	0.0
Net Transfers In (Out)	0.0
Fair value, ending balance	3.0
Nonredeemable preferred stocks \| Financials
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Fair value, beginning balance	49.2	67.4
Calls/ Maturities/ Paydowns	0.0	0.0
Purchases	0.0	0.0
Sales	0.0	0.0
Net Realized (Gain)/Loss on Sales	0.0	0.0
Change in Valuation	0.0	0.0
Net Transfers In (Out)	0.0	0.0
Fair value, ending balance	49.2	67.4
Nonredeemable preferred stocks \| Industrials
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Fair value, beginning balance	14.8	16.4
Calls/ Maturities/ Paydowns	0.0	0.0
Purchases	0.0	0.0
Sales	0.0	0.0
Net Realized (Gain)/Loss on Sales	0.0	0.0
Change in Valuation	0.0	0.0
Net Transfers In (Out)	0.0	0.0
Fair value, ending balance	14.8	16.4
Common equities \| Common Stocks, excluding Other Risk
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Fair value, beginning balance	22.5	18.3
Calls/ Maturities/ Paydowns	0.0	0.0
Purchases	0.0	0.0
Sales	0.0	0.0
Net Realized (Gain)/Loss on Sales	0.0	0.0
Change in Valuation	0.0	0.0
Net Transfers In (Out)	0.0	0.0
Fair value, ending balance	22.5	18.3
Common equities \| Other risk investments
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Fair value, beginning balance	20.6	19.8
Calls/ Maturities/ Paydowns	3.7	0.5
Purchases	0.0	0.0
Sales	0.0	0.0
Net Realized (Gain)/Loss on Sales	0.0	0.0
Change in Valuation	0.0	0.0
Net Transfers In (Out)	0.0	0.0
Fair value, ending balance	$ 24.3	$ 20.3

A roll forward is a reconciliation of a concept from the beginning of a period to the end of a period.

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- Definition

Amount of gain (loss) recognized in income from asset measured at fair value on recurring basis using unobservable input (level 3).

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Topic 820
-SubTopic 10
-Name Accounting Standards Codification
-Section 50
-Paragraph 2
-Subparagraph (c)(1)
-Publisher FASB
-URI https://asc.fasb.org//1943274/2147482106/820-10-50-2

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- Definition

Amount of increase (decrease) of financial instrument classified as an asset measured using unobservable inputs that reflect the entity's own assumption about the assumptions market participants would use in pricing.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
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-Paragraph 2
-Subparagraph (c)
-SubTopic 10
-Topic 820
-Publisher FASB
-URI https://asc.fasb.org//1943274/2147482106/820-10-50-2

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- Definition

Amount of purchases of financial instrument classified as an asset measured using unobservable inputs that reflect the entity's own assumption about the assumptions market participants would use in pricing.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Name Accounting Standards Codification
-Topic 820
-SubTopic 10
-Section 50
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-Publisher FASB
-URI https://asc.fasb.org//1943274/2147482106/820-10-50-2

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- Definition

Amount of sale of financial instrument classified as an asset measured using unobservable inputs that reflect the entity's own assumption about the assumptions market participants would use in pricing.

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- Definition

Amount of transfer of financial instrument classified as an asset into (out of) level 3 of the fair value hierarchy.

+ References

No definition available.

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