Investments - Components of Net Investment Income (Details) - USD ($) $ in Millions	3 Months Ended	9 Months Ended
Sep. 30, 2023	Sep. 30, 2022	Sep. 30, 2023	Sep. 30, 2022
Net Investment Income [Line Items]
Gross investment income	$ 510.2	$ 333.6	$ 1,384.3	$ 868.2
Investment expenses	(7.2)	(5.8)	(18.8)	(17.4)
Net Investment Income	503.0	327.8	1,365.5	850.8
Available-for-sale Securities
Net Investment Income [Line Items]
Gross investment income	487.6	304.8	1,311.1	766.2
Total equity securities
Net Investment Income [Line Items]
Gross investment income	22.6	28.8	73.2	102.0
Total equity securities \| Nonredeemable preferred stocks
Net Investment Income [Line Items]
Gross investment income	11.9	17.3	40.3	53.6
Total equity securities \| Common equities
Net Investment Income [Line Items]
Gross investment income	10.7	11.5	32.9	48.4
Total fixed maturities
Net Investment Income [Line Items]
Gross investment income	466.0	280.0	1,240.0	736.6
Total fixed maturities \| U.S. government obligations
Net Investment Income [Line Items]
Gross investment income	237.0	86.1	591.7	205.4
Total fixed maturities \| State and local government obligations
Net Investment Income [Line Items]
Gross investment income	12.3	9.8	34.5	29.3
Total fixed maturities \| Foreign government obligations
Net Investment Income [Line Items]
Gross investment income	0.1	0.1	0.2	0.2
Total fixed maturities \| Corporate debt securities
Net Investment Income [Line Items]
Gross investment income	94.8	71.8	261.9	215.4
Total fixed maturities \| Residential mortgage-backed securities
Net Investment Income [Line Items]
Gross investment income	7.6	8.8	22.6	23.2
Total fixed maturities \| Commercial mortgage-backed securities
Net Investment Income [Line Items]
Gross investment income	48.7	48.3	147.9	138.1
Total fixed maturities \| Other asset-backed securities
Net Investment Income [Line Items]
Gross investment income	62.9	52.1	173.0	116.5
Total fixed maturities \| Redeemable preferred stocks
Net Investment Income [Line Items]
Gross investment income	2.6	3.0	8.2	8.5
Short-term investments
Net Investment Income [Line Items]
Gross investment income	$ 21.6	$ 24.8	$ 71.1	$ 29.6

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 of expenses related to the generation of investment income.

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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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