Fair Value - Summary of Changes in Fair Value Associated with Level 3 Assets (Details) - USD ($) $ in Millions	3 Months Ended	9 Months Ended
Sep. 30, 2023	Sep. 30, 2022	Sep. 30, 2023	Sep. 30, 2022
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Fair value, beginning balance	$ 112.0	$ 100.4	$ 121.9	$ 127.7
Calls/ Maturities/ Paydowns	0.7	0.2	2.8	1.3
Purchases	0.0	0.0	0.0	0.0
Sales	(10.0)	0.0	(10.0)	(15.0)
Net Realized (Gain)/Loss on Sales	8.9	0.0	8.9	(17.2)
Change in Valuation	(7.0)	0.0	(19.0)	3.8
Net Transfers In (Out)	0.0	0.0	0.0	0.0
Fair value, ending balance	104.6	100.6	104.6	100.6
Total equity securities \| Financials \| Nonredeemable preferred stocks
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Fair value, beginning balance	56.0	64.9	67.4	76.4
Calls/ Maturities/ Paydowns	(8.1)	0.0	(8.1)	0.0
Purchases	0.0	0.0	0.0	0.0
Sales	0.0	0.0	0.0	(15.0)
Net Realized (Gain)/Loss on Sales	0.0	0.0	0.0	(17.2)
Change in Valuation	0.0	0.0	(11.4)	20.7
Net Transfers In (Out)	0.0	0.0	0.0	0.0
Fair value, ending balance	47.9	64.9	47.9	64.9
Total equity securities \| Industrials \| Nonredeemable preferred stocks
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Fair value, beginning balance	15.8	17.0	16.4	34.4
Calls/ Maturities/ Paydowns	0.0	0.0	0.0	(0.5)
Purchases	0.0	0.0	0.0	0.0
Sales	0.0	0.0	0.0	0.0
Net Realized (Gain)/Loss on Sales	0.0	0.0	0.0	0.0
Change in Valuation	0.0	0.0	(0.6)	(16.9)
Net Transfers In (Out)	0.0	0.0	0.0	0.0
Fair value, ending balance	15.8	17.0	15.8	17.0
Total equity securities \| Common Stocks, excluding Other Risk \| Common equities
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Fair value, beginning balance	18.3	0.0	18.3
Calls/ Maturities/ Paydowns	8.1	0.0	8.1
Purchases	0.0	0.0	0.0
Sales	(10.0)	0.0	(10.0)
Net Realized (Gain)/Loss on Sales	8.9	0.0	8.9
Change in Valuation	(7.0)	0.0	(7.0)
Net Transfers In (Out)	0.0	0.0	0.0
Fair value, ending balance	18.3	0.0	18.3	0.0
Total equity securities \| Other risk investments \| Common equities
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Fair value, beginning balance	21.9	18.5	19.8	16.9
Calls/ Maturities/ Paydowns	0.7	0.2	2.8	1.8
Purchases	0.0	0.0	0.0	0.0
Sales	0.0	0.0	0.0	0.0
Net Realized (Gain)/Loss on Sales	0.0	0.0	0.0	0.0
Change in Valuation	0.0	0.0	0.0	0.0
Net Transfers In (Out)	0.0	0.0	0.0	0.0
Fair value, ending balance	$ 22.6	$ 18.7	$ 22.6	$ 18.7

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)
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-Topic 820
-Publisher FASB
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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
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-SubTopic 10
-Section 50
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-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.

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