SCHEDULE I SUMMARY OF INVESTMENTS - OTHER THAN INVESTMENTS IN RELATED PARTIES (Details) $ in Millions	Dec. 31, 2018 USD ($)
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]
Cost	$ 7,297.7	[1]
Fair Value	7,316.4
Amount at which shown in the balance sheet	7,311.9
US Treasury and Government Agencies [Member]
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]
Cost	1,140.2	[1]
Fair Value	1,123.6
Amount at which shown in the balance sheet	1,123.6
States, municipalities and political subdivisions [Member]
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]
Cost	879.0	[1]
Fair Value	885.8
Amount at which shown in the balance sheet	885.8
Foreign governments [Member]
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]
Cost	7.3	[1]
Fair Value	7.4
Amount at which shown in the balance sheet	7.4
Public Utilities Bonds [Member]
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]
Cost	410.3	[1]
Fair Value	403.1
Amount at which shown in the balance sheet	403.1
All other corporate bonds [Member]
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]
Cost	3,809.1	[1]
Fair Value	3,741.6
Amount at which shown in the balance sheet	3,741.6
Fixed Maturities [Member]
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]
Cost	6,245.9	[1]
Fair Value	6,161.5
Amount at which shown in the balance sheet	6,161.5
Public utilities [Member]
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]
Cost	63.9	[1]
Fair Value	90.7
Amount at which shown in the balance sheet	90.7
Banks, trust and insurance companies [Member]
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]
Cost	28.5	[1]
Fair Value	27.4
Amount at which shown in the balance sheet	27.4
Industrial, miscellaneous and all other [Member]
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]
Cost	290.6	[1]
Fair Value	346.3
Amount at which shown in the balance sheet	346.3
Equity Securities Investment Summary [Member]
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]
Cost	383.0	[1]
Fair Value	464.4
Amount at which shown in the balance sheet	464.4
Mortgage Loans on Real Estate [Member]
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]
Cost	405.7	[1]
Fair Value	410.2
Amount at which shown in the balance sheet	405.7
Real estate [Member]
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]
Cost	7.9	[1]
Fair Value	7.9
Amount at which shown in the balance sheet	7.9
Other long-term investments [Member]
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]
Cost	230.3	[1]
Fair Value	247.9	[1]
Amount at which shown in the balance sheet	247.9	[1]
Total investments, excluding held-for-sale [Member]
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]
Cost	7,272.8	[1]
Fair Value	7,291.9
Amount at which shown in the balance sheet	7,287.4
Assets Held-for-Sale [Member] | Chaucer [Member]
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]
Cost	24.9	[1]
Fair Value	24.5
Amount at which shown in the balance sheet	$ 24.5

[1] For equity securities and other long-term investments, represents original cost; for fixed maturities, represents original cost reduced by repayments and adjusted for amortization of premiums and accretion of discounts.