Schedule I - Summary of Investments - Other Than Investments in Related Parties (Details) - USD ($) $ in Millions		Dec. 31, 2020	Dec. 31, 2019
Summary of Investments, Other than Investments in Related Parties, Reportable Data
Available-for-sale Securities, Amortized Cost Basis		$ 10,647.3
Available-for-sale Securities		11,246.4	$ 9,280.9
Investment Owned, at Cost	[1]	13,945.9
Investments		15,330.1	$ 14,359.2
Short-term Investments [Member]
Summary of Investments, Other than Investments in Related Parties, Reportable Data
Cost	[1]	749.6
Amount at which shown in balance sheet		749.6
Other investments [Member]
Summary of Investments, Other than Investments in Related Parties, Reportable Data
Cost	[1]	28.8
Amount at which shown in balance sheet		28.8
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]	1,801.5
Fair Value		1,891.4
Amount at which shown in balance sheet		1,891.4
Fixed maturity securities [Member] | States, municipalities and political subdivisions [Member]
Summary of Investments, Other than Investments in Related Parties, Reportable Data
Cost	[1]	997.1
Fair Value		1,063.5
Amount at which shown in balance sheet		1,063.5
Fixed maturity securities [Member] | Foreign government [Member]
Summary of Investments, Other than Investments in Related Parties, Reportable Data
Cost	[1]	165.5
Fair Value		171.8
Amount at which shown in balance sheet		171.8
Fixed maturity securities [Member] | Corporate, industrial and all other [Member]
Summary of Investments, Other than Investments in Related Parties, Reportable Data
Cost	[1]	6,933.3
Fair Value		7,370.0
Amount at which shown in balance sheet		7,370.0
Fixed maturity securities [Member] | Fixed Maturities [Member]
Summary of Investments, Other than Investments in Related Parties, Reportable Data
Cost	[1]	9,897.6
Fair Value		10,496.8
Amount at which shown in balance sheet		10,496.8
Equity Securities [Member] | Non-redeemable preferred stock [Member]
Summary of Investments, Other than Investments in Related Parties, Reportable Data
Cost	[1]	0.6
Fair Value		1.2
Amount at which shown in balance sheet		1.2
Equity Securities [Member] | Banks, trust and insurance companies [Member]
Summary of Investments, Other than Investments in Related Parties, Reportable Data
Cost	[1]	111.0
Fair Value		184.6
Amount at which shown in balance sheet		184.6
Equity Securities [Member] | Industrial, miscellaneous, and all others [Member]
Summary of Investments, Other than Investments in Related Parties, Reportable Data
Cost	[1]	3,108.2
Fair Value		3,814.2
Amount at which shown in balance sheet		3,814.2
Equity Securities [Member] | Indexed mutual funds [Member]
Summary of Investments, Other than Investments in Related Parties, Reportable Data
Cost	[1]	49.8
Fair Value		54.6
Amount at which shown in balance sheet		54.6
Equity Securities [Member] | Equity Securities [Member]
Summary of Investments, Other than Investments in Related Parties, Reportable Data
Cost	[1]	3,269.7
Fair Value		4,054.8
Amount at which shown in balance sheet		$ 4,054.8

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