Schedule I - Summary of Investments - Other Than Investments in Related Parties (Details) - USD ($) $ in Millions		Dec. 31, 2023	Dec. 31, 2022
Summary of Investments, Other than Investments in Related Parties, Reportable Data
Investment Owned, at Cost	[1]	$ 14,841.8
Investments		15,867.7	$ 15,859.9
Short-term Investments [Member]
Summary of Investments, Other than Investments in Related Parties, Reportable Data
Cost	[1]	1,032.6
Amount at which shown in balance sheet		1,032.6
Other investments [Member]
Summary of Investments, Other than Investments in Related Parties, Reportable Data
Cost	[1]	34.3
Amount at which shown in balance sheet		34.3
Fixed maturity securities [Member] | United States Government and government agencies and authorities [Member]
Summary of Investments, Other than Investments in Related Parties, Reportable Data
Cost	[1]	1,695.3
Fair Value		1,641.5
Amount at which shown in balance sheet		1,641.5
Fixed maturity securities [Member] | States, municipalities and political subdivisions [Member]
Summary of Investments, Other than Investments in Related Parties, Reportable Data
Cost	[1]	774.5
Fair Value		767.6
Amount at which shown in balance sheet		767.6
Fixed maturity securities [Member] | Foreign government [Member]
Summary of Investments, Other than Investments in Related Parties, Reportable Data
Cost	[1]	224.9
Fair Value		217.3
Amount at which shown in balance sheet		217.3
Fixed maturity securities [Member] | Corporate, industrial and all other [Member]
Summary of Investments, Other than Investments in Related Parties, Reportable Data
Cost	[1]	9,568.1
Fair Value		9,513.3
Amount at which shown in balance sheet		9,513.3
Fixed maturity securities [Member] | Fixed Maturities [Member]
Summary of Investments, Other than Investments in Related Parties, Reportable Data
Cost	[1]	12,263.0
Fair Value		12,139.9
Amount at which shown in balance sheet		12,139.9
Equity Securities [Member] | Non-redeemable preferred stock [Member]
Summary of Investments, Other than Investments in Related Parties, Reportable Data
Cost	[1]	5.6
Fair Value		6.2
Amount at which shown in balance sheet		6.2
Equity Securities [Member] | Banks, trust and insurance companies [Member]
Summary of Investments, Other than Investments in Related Parties, Reportable Data
Cost	[1]	79.0
Fair Value		163.7
Amount at which shown in balance sheet		163.7
Equity Securities [Member] | Industrial, miscellaneous, and all others [Member]
Summary of Investments, Other than Investments in Related Parties, Reportable Data
Cost	[1]	1,427.2
Fair Value		2,490.8
Amount at which shown in balance sheet		2,490.8
Equity Securities [Member] | Equity Securities [Member]
Summary of Investments, Other than Investments in Related Parties, Reportable Data
Cost	[1]	1,511.9
Fair Value		2,660.8
Amount at which shown in balance sheet		2,660.8
Fixed Maturity and Short-Term Investments
Summary of Investments, Other than Investments in Related Parties, Reportable Data
Cost		13,295.6
Amount at which shown in balance sheet		$ 13,172.5

[1] (a)    Represents original cost of equity securities, and as to fixed incomes, original cost reduced by repayments and adjusted for amortization of premium or accrual of discount.