Fair Value - Summary of Changes in Fair Value Associated with Level 3 Assets (Details) - USD ($) $ in Millions	3 Months Ended	6 Months Ended
Jun. 30, 2025	Jun. 30, 2024	Jun. 30, 2025	Jun. 30, 2024
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Fair value, beginning balance	$ 105.0	$ 114.0	$ 105.0	$ 110.0
Calls/ Maturities/ Paydowns/Other	1.0	(1.0)	7.0	3.0
Purchases	0.0	0.0	8.0	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 Valuation1	0.0	(12.0)	(14.0)	(12.0)
Net Transfers In (Out)	0.0	0.0	0.0	0.0
Fair value, ending balance	106.0	101.0	106.0	101.0
Corporate and other debt securities
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Fair value, beginning balance	5.0	3.0	5.0	3.0
Calls/ Maturities/ Paydowns/Other	0.0	0.0	0.0	0.0
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 Valuation1	0.0	0.0	0.0	0.0
Net Transfers In (Out)	0.0	0.0	0.0	0.0
Fair value, ending balance	5.0	3.0	5.0	3.0
Nonredeemable preferred stocks
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Fair value, beginning balance	60.0	64.0	52.0	64.0
Calls/ Maturities/ Paydowns/Other	0.0	0.0	0.0	0.0
Purchases	0.0	0.0	8.0	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 Valuation1	0.0	(12.0)	0.0	(12.0)
Net Transfers In (Out)	0.0	0.0	0.0	0.0
Fair value, ending balance	60.0	52.0	60.0	52.0
Common equities \| Common Stocks, excluding Other Risk
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Fair value, beginning balance	9.0	22.0	23.0	22.0
Calls/ Maturities/ Paydowns/Other	0.0	0.0	0.0	0.0
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 Valuation1	0.0	0.0	(14.0)	0.0
Net Transfers In (Out)	0.0	0.0	0.0	0.0
Fair value, ending balance	9.0	22.0	9.0	22.0
Common equities \| Other risk investments
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Fair value, beginning balance	31.0	25.0	25.0	21.0
Calls/ Maturities/ Paydowns/Other	1.0	(1.0)	7.0	3.0
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 Valuation1	0.0	0.0	0.0	0.0
Net Transfers In (Out)	0.0	0.0	0.0	0.0
Fair value, ending balance	$ 32.0	$ 24.0	$ 32.0	$ 24.0

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).

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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.

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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.

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

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