Investments - Components of Net Investment Income (Details) - USD ($) $ in Millions	3 Months Ended	6 Months Ended
Jun. 30, 2024	Jun. 30, 2023	Jun. 30, 2024	Jun. 30, 2023
Net Investment Income [Line Items]
Gross investment income	$ 685.0	$ 454.5	$ 1,302.6	$ 874.1
Investment expenses	(7.3)	(6.1)	(13.0)	(11.6)
Net Investment Income	677.7	448.4	1,289.6	862.5
Available-for-sale Securities
Net Investment Income [Line Items]
Gross investment income	664.8	430.7	1,257.5	823.5
Total equity securities
Net Investment Income [Line Items]
Gross investment income	20.2	23.8	45.1	50.6
Total equity securities \| Nonredeemable preferred stocks
Net Investment Income [Line Items]
Gross investment income	10.4	13.3	21.4	28.4
Total equity securities \| Common equities
Net Investment Income [Line Items]
Gross investment income	9.8	10.5	23.7	22.2
Total fixed maturities
Net Investment Income [Line Items]
Gross investment income	647.6	405.9	1,221.4	774.0
Total fixed maturities \| U.S. government obligations
Net Investment Income [Line Items]
Gross investment income	361.3	191.5	668.2	354.7
Total fixed maturities \| State and local government obligations
Net Investment Income [Line Items]
Gross investment income	13.4	11.6	25.8	22.2
Total fixed maturities \| Foreign government obligations
Net Investment Income [Line Items]
Gross investment income	0.0	0.0	0.1	0.1
Total fixed maturities \| Corporate debt securities
Net Investment Income [Line Items]
Gross investment income	134.8	83.5	256.3	167.1
Total fixed maturities \| Residential mortgage-backed securities
Net Investment Income [Line Items]
Gross investment income	8.0	5.8	13.4	15.0
Total fixed maturities \| Commercial mortgage-backed securities
Net Investment Income [Line Items]
Gross investment income	46.1	49.2	92.5	99.2
Total fixed maturities \| Other asset-backed securities
Net Investment Income [Line Items]
Gross investment income	81.3	61.6	159.5	110.1
Total fixed maturities \| Redeemable preferred stocks
Net Investment Income [Line Items]
Gross investment income	2.7	2.7	5.6	5.6
Short-term investments
Net Investment Income [Line Items]
Gross investment income	$ 17.2	$ 24.8	$ 36.1	$ 49.5

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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