Schedule 1 - Investments Other Than Investments in Related Parties (Details) $ in Millions	Dec. 31, 2019 USD ($)
SEC Schedule, 12-15, Insurance Companies, Summary of Investments, Other than Investments in Related Parties [Line Items]
Amortized Cost	$ 8,495.0
Amount Carried in Balance Sheet	9,044.4
Fixed Maturities:
SEC Schedule, 12-15, Insurance Companies, Summary of Investments, Other than Investments in Related Parties [Line Items]
Amortized Cost	6,372.7
Fair Value	6,922.1
Amount Carried in Balance Sheet	6,922.1
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	784.7
Fair Value	815.9
Amount Carried in Balance Sheet	815.9
States and Political Subdivisions
SEC Schedule, 12-15, Insurance Companies, Summary of Investments, Other than Investments in Related Parties [Line Items]
Amortized Cost	1,386.4
Fair Value	1,515.8
Amount Carried in Balance Sheet	1,515.8
Foreign Governments
SEC Schedule, 12-15, Insurance Companies, Summary of Investments, Other than Investments in Related Parties [Line Items]
Amortized Cost	17.2
Fair Value	16.8
Amount Carried in Balance Sheet	16.8
Other Bonds and Notes
SEC Schedule, 12-15, Insurance Companies, Summary of Investments, Other than Investments in Related Parties [Line Items]
Amortized Cost	3,465.0
Fair Value	3,859.7
Amount Carried in Balance Sheet	3,859.7
Redeemable Preferred Stocks
SEC Schedule, 12-15, Insurance Companies, Summary of Investments, Other than Investments in Related Parties [Line Items]
Amortized Cost	6.8
Fair Value	6.7
Amount Carried in Balance Sheet	6.7
Collateralized Loan Obligations
SEC Schedule, 12-15, Insurance Companies, Summary of Investments, Other than Investments in Related Parties [Line Items]
Amortized Cost	624.6
Fair Value	618.2
Amount Carried in Balance Sheet	618.2
Other Mortgage- and Asset-backed
SEC Schedule, 12-15, Insurance Companies, Summary of Investments, Other than Investments in Related Parties [Line Items]
Amortized Cost	88.0
Fair Value	89.0
Amount Carried in Balance Sheet	89.0
Equity Securities at Fair Value:
SEC Schedule, 12-15, Insurance Companies, Summary of Investments, Other than Investments in Related Parties [Line Items]
Amortized Cost	907.3
Fair Value	907.3
Amount Carried in Balance Sheet	907.3
Preferred Stocks
SEC Schedule, 12-15, Insurance Companies, Summary of Investments, Other than Investments in Related Parties [Line Items]
Amortized Cost	59.2
Fair Value	59.2
Amount Carried in Balance Sheet	59.2
Common Stock
SEC Schedule, 12-15, Insurance Companies, Summary of Investments, Other than Investments in Related Parties [Line Items]
Amortized Cost	13.2
Fair Value	13.2
Amount Carried in Balance Sheet	13.2
Other Equity Interests
SEC Schedule, 12-15, Insurance Companies, Summary of Investments, Other than Investments in Related Parties [Line Items]
Amortized Cost	834.9
Fair Value	834.9
Amount Carried in Balance Sheet	834.9
Equity Securities at Modified Cost
SEC Schedule, 12-15, Insurance Companies, Summary of Investments, Other than Investments in Related Parties [Line Items]
Amortized Cost	41.9
Amount Carried in Balance Sheet	41.9
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	220.4
Amount Carried in Balance Sheet	220.4
Convertible Securities at Fair Value
SEC Schedule, 12-15, Insurance Companies, Summary of Investments, Other than Investments in Related Parties [Line Items]
Amortized Cost	37.3
Fair Value	37.3
Amount Carried in Balance Sheet	37.3
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	444.5
Amount Carried in Balance Sheet	444.5
Short-term Investments
SEC Schedule, 12-15, Insurance Companies, Summary of Investments, Other than Investments in Related Parties [Line Items]
Amortized Cost	470.9
Amount Carried in Balance Sheet	$ 470.9