Investments - Components of Net Investment Income (Details) - USD ($) $ in Millions	3 Months Ended	6 Months Ended
Jun. 30, 2025	Jun. 30, 2024	Jun. 30, 2025	Jun. 30, 2024
Net Investment Income [Line Items]
Gross investment income	$ 871.0	$ 685.0	$ 1,685.0	$ 1,303.0
Investment expenses	(9.0)	(7.0)	(16.0)	(13.0)
Net Investment Income	862.0	678.0	1,669.0	1,290.0
Available-for-sale Securities
Net Investment Income [Line Items]
Gross investment income	854.0	665.0	1,646.0	1,258.0
Total equity securities
Net Investment Income [Line Items]
Gross investment income	17.0	20.0	39.0	45.0
Total equity securities \| Nonredeemable preferred stocks
Net Investment Income [Line Items]
Gross investment income	5.0	10.0	13.0	21.0
Total equity securities \| Common equities
Net Investment Income [Line Items]
Gross investment income	12.0	10.0	26.0	24.0
Total fixed maturities
Net Investment Income [Line Items]
Gross investment income	826.0	648.0	1,600.0	1,222.0
Total fixed maturities \| U.S. government obligations
Net Investment Income [Line Items]
Gross investment income	422.0	361.0	844.0	668.0
Total fixed maturities \| State and local government obligations
Net Investment Income [Line Items]
Gross investment income	21.0	13.0	40.0	26.0
Total fixed maturities \| Corporate and other debt securities
Net Investment Income [Line Items]
Gross investment income	208.0	137.0	379.0	262.0
Total fixed maturities \| Residential mortgage-backed securities
Net Investment Income [Line Items]
Gross investment income	32.0	8.0	57.0	13.0
Total fixed maturities \| Commercial mortgage-backed securities
Net Investment Income [Line Items]
Gross investment income	59.0	47.0	112.0	93.0
Total fixed maturities \| Other asset-backed securities
Net Investment Income [Line Items]
Gross investment income	84.0	82.0	168.0	160.0
Short-term investments
Net Investment Income [Line Items]
Gross investment income	$ 28.0	$ 17.0	$ 46.0	$ 36.0

Amount, before investment expense, of income earned from investments in securities and real estate. Includes, but is not limited to, real estate investment, policy loans, dividends, and interest. Excludes realized gain (loss) on investments.

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Amount, after investment expense, of income earned from investments in securities and real estate. Includes, but is not limited to, real estate investment, policy loans, dividends, and interest. Excludes realized gain (loss) on investments.

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