SCHEDULE I SUMMARY OF INVESTMENTS - OTHER THAN INVESTMENTS IN RELATED PARTIES (Details) $ in Millions	Dec. 31, 2023 USD ($)
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]
Cost	$ 9,389.7	[1]
Fair Value	8,893.5
Amount at which shown in the balance sheet	8,913.1	[2]
U.S. Treasury and Government Agencies [Member]
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]
Cost	1,798.2	[1]
Fair Value	1,626.8
Amount at which shown in the balance sheet	1,626.8	[2]
States, municipalities and political subdivisions [Member]
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]
Cost	1,306.0	[1]
Fair Value	1,181.9
Amount at which shown in the balance sheet	1,181.9	[2]
Foreign governments [Member]
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]
Cost	2.2	[1]
Fair Value	2.2
Amount at which shown in the balance sheet	2.2	[2]
Public Utilities Bonds [Member]
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]
Cost	12.7	[1]
Fair Value	25.0
Amount at which shown in the balance sheet	25.0	[2]
All other corporate bonds [Member]
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]
Cost	5,106.9	[1]
Fair Value	4,831.8
Amount at which shown in the balance sheet	4,831.8	[2]
Fixed Maturities [Member]
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]
Cost	8,563.5	[1]
Fair Value	7,973.0
Amount at which shown in the balance sheet	7,973.0	[2]
Public utilities [Member]
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]
Cost	350.2	[1]
Fair Value	330.3
Amount at which shown in the balance sheet	330.3	[2]
Banks, trust and insurance companies [Member]
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]
Cost	1.6	[1]
Fair Value	2.4
Amount at which shown in the balance sheet	2.4	[2]
Industrial, miscellaneous and all other [Member]
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]
Cost	38.9	[1]
Fair Value	97.0
Amount at which shown in the balance sheet	97.0	[2]
Nonredeemable preferred stock [Member]
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]
Cost	1.0	[1]
Fair Value	6.5
Amount at which shown in the balance sheet	6.5	[2]
Equity Securities Investment Summary [Member]
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]
Cost	54.2	[1]
Fair Value	130.9
Amount at which shown in the balance sheet	130.9	[2]
Mortgage Loans on Real Estate [Member]
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]
Cost	381.4	[1]
Fair Value	351.8
Amount at which shown in the balance sheet	371.4	[2]
Other long-term investments [Member]
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]
Cost	378.3	[1]
Fair Value	425.5
Amount at which shown in the balance sheet	425.5	[2]
Short-Term Investments [Member]
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]
Cost	12.3	[1]
Fair Value	12.3
Amount at which shown in the balance sheet	$ 12.3	[2]

[1] Original cost of equity securities and, as to fixed maturities, original cost reduced by repayments and adjusted for amortization of premiums and accretion of discounts.

[2] Mortgage loans on real estate are shown on the balance sheet net of the allowance for credit losses.