SCHEDULE I SUMMARY OF INVESTMENTS - OTHER THAN INVESTMENTS IN RELATED PARTIES (Details) $ in Millions	Dec. 31, 2020 USD ($)
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]
Cost	$ 8,131.9	[1]
Fair Value	8,880.6
Amount at which shown in the balance sheet	8,846.1	[2]
U.S. Treasury and Government Agencies [Member]
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]
Cost	1,353.1	[1]
Fair Value	1,401.1
Amount at which shown in the balance sheet	1,401.1	[2]
States, municipalities and political subdivisions [Member]
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]
Cost	1,042.1	[1]
Fair Value	1,103.7
Amount at which shown in the balance sheet	1,103.7	[2]
Foreign governments [Member]
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]
Cost	2.2	[1]
Fair Value	2.7
Amount at which shown in the balance sheet	2.7	[2]
Public Utilities Bonds [Member]
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]
Cost	392.3	[1]
Fair Value	430.1
Amount at which shown in the balance sheet	430.1	[2]
All other corporate bonds [Member]
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]
Cost	4,156.0	[1]
Fair Value	4,516.8
Amount at which shown in the balance sheet	4,516.8	[2]
Fixed Maturities [Member]
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]
Cost	6,945.7	[1]
Fair Value	7,454.4
Amount at which shown in the balance sheet	7,454.4	[2]
Public utilities [Member]
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]
Cost	56.8	[1]
Fair Value	89.6
Amount at which shown in the balance sheet	89.6	[2]
Banks, trust and insurance companies [Member]
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]
Cost	44.1	[1]
Fair Value	47.6
Amount at which shown in the balance sheet	47.6	[2]
Industrial, miscellaneous and all other [Member]
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]
Cost	302.0	[1]
Fair Value	456.9
Amount at which shown in the balance sheet	456.9	[2]
Nonredeemable preferred stock [Member]
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]
Cost	1.0	[1]
Fair Value	4.4
Amount at which shown in the balance sheet	4.4	[2]
Equity Securities Investment Summary [Member]
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]
Cost	403.9	[1]
Fair Value	598.5
Amount at which shown in the balance sheet	598.5	[2]
Mortgage Loans on Real Estate [Member]
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]
Cost	475.5	[1]
Fair Value	502.1
Amount at which shown in the balance sheet	467.6	[2]
Real estate [Member]
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]
Cost	6.4	[1]
Fair Value	6.4
Amount at which shown in the balance sheet	6.4	[2]
Other long-term investments [Member]
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]
Cost	300.4	[1]
Fair Value	319.2	[1]
Amount at which shown in the balance sheet	$ 319.2	[1],[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.