Schedule 1 - Investments Other Than Investments in Related Parties (Details) $ in Millions	Dec. 31, 2020 USD ($)
SEC Schedule, 12-15, Insurance Companies, Summary of Investments, Other than Investments in Related Parties [Line Items]
Amortized Cost	$ 9,510.9
Amount Carried in Balance Sheet	10,424.1
Fixed Maturities:
SEC Schedule, 12-15, Insurance Companies, Summary of Investments, Other than Investments in Related Parties [Line Items]
Amortized Cost	6,692.7
Fair Value	7,605.9
Amount Carried in Balance Sheet	7,605.9
United States Government and Government Agencies and Authorities
SEC Schedule, 12-15, Insurance Companies, Summary of Investments, Other than Investments in Related Parties [Line Items]
Amortized Cost	536.5
Fair Value	585.3
Amount Carried in Balance Sheet	585.3
States and Political Subdivisions
SEC Schedule, 12-15, Insurance Companies, Summary of Investments, Other than Investments in Related Parties [Line Items]
Amortized Cost	1,404.3
Fair Value	1,589.5
Amount Carried in Balance Sheet	1,589.5
Foreign Governments
SEC Schedule, 12-15, Insurance Companies, Summary of Investments, Other than Investments in Related Parties [Line Items]
Amortized Cost	6.6
Fair Value	5.2
Amount Carried in Balance Sheet	5.2
Other Bonds and Notes
SEC Schedule, 12-15, Insurance Companies, Summary of Investments, Other than Investments in Related Parties [Line Items]
Amortized Cost	3,749.5
Fair Value	4,425.4
Amount Carried in Balance Sheet	4,425.4
Redeemable Preferred Stocks
SEC Schedule, 12-15, Insurance Companies, Summary of Investments, Other than Investments in Related Parties [Line Items]
Amortized Cost	7.0
Fair Value	7.5
Amount Carried in Balance Sheet	7.5
Collateralized Loan Obligations
SEC Schedule, 12-15, Insurance Companies, Summary of Investments, Other than Investments in Related Parties [Line Items]
Amortized Cost	785.1
Fair Value	767.7
Amount Carried in Balance Sheet	767.7
Other Mortgage- and Asset-backed
SEC Schedule, 12-15, Insurance Companies, Summary of Investments, Other than Investments in Related Parties [Line Items]
Amortized Cost	203.7
Fair Value	225.3
Amount Carried in Balance Sheet	225.3
Equity Securities at Fair Value:
SEC Schedule, 12-15, Insurance Companies, Summary of Investments, Other than Investments in Related Parties [Line Items]
Amortized Cost	858.5
Fair Value	858.5
Amount Carried in Balance Sheet	858.5
Preferred Stocks
SEC Schedule, 12-15, Insurance Companies, Summary of Investments, Other than Investments in Related Parties [Line Items]
Amortized Cost	59.1
Fair Value	59.1
Amount Carried in Balance Sheet	59.1
Common Stock
SEC Schedule, 12-15, Insurance Companies, Summary of Investments, Other than Investments in Related Parties [Line Items]
Amortized Cost	10.8
Fair Value	10.8
Amount Carried in Balance Sheet	10.8
Other Equity Interests
SEC Schedule, 12-15, Insurance Companies, Summary of Investments, Other than Investments in Related Parties [Line Items]
Amortized Cost	788.6
Fair Value	788.6
Amount Carried in Balance Sheet	788.6
Equity Securities at Modified Cost
SEC Schedule, 12-15, Insurance Companies, Summary of Investments, Other than Investments in Related Parties [Line Items]
Amortized Cost	40.1
Amount Carried in Balance Sheet	40.1
Equity Method Limited Liability Investments at Cost Plus Cumulative Undistributed Earnings
SEC Schedule, 12-15, Insurance Companies, Summary of Investments, Other than Investments in Related Parties [Line Items]
Amortized Cost	225.3
Amount Carried in Balance Sheet	225.3
Convertible Securities at Fair Value
SEC Schedule, 12-15, Insurance Companies, Summary of Investments, Other than Investments in Related Parties [Line Items]
Amortized Cost	39.9
Fair Value	39.9
Amount Carried in Balance Sheet	39.9
Loans, Real Estate and Other Investments
SEC Schedule, 12-15, Insurance Companies, Summary of Investments, Other than Investments in Related Parties [Line Items]
Amortized Cost	779.0
Amount Carried in Balance Sheet	779.0
Short-term Investments
SEC Schedule, 12-15, Insurance Companies, Summary of Investments, Other than Investments in Related Parties [Line Items]
Amortized Cost	875.4
Amount Carried in Balance Sheet	$ 875.4