Schedule 1 - Investments Other Than Investments in Related Parties (Details) $ in Millions	Dec. 31, 2023 USD ($)
SEC Schedule, 12-15, Insurance Companies, Summary of Investments, Other than Investments in Related Parties [Line Items]
Amortized Cost	$ 9,588.1
Amount Carried in Balance Sheet	8,904.2
Fixed Maturities:
SEC Schedule, 12-15, Insurance Companies, Summary of Investments, Other than Investments in Related Parties [Line Items]
Amortized Cost	7,565.8
Fair Value	6,881.9
Amount Carried in Balance Sheet	6,881.9
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	594.1
Fair Value	511.5
Amount Carried in Balance Sheet	511.5
States and Political Subdivisions
SEC Schedule, 12-15, Insurance Companies, Summary of Investments, Other than Investments in Related Parties [Line Items]
Amortized Cost	1,575.9
Fair Value	1,401.9
Amount Carried in Balance Sheet	1,401.9
Foreign Governments
SEC Schedule, 12-15, Insurance Companies, Summary of Investments, Other than Investments in Related Parties [Line Items]
Amortized Cost	4.4
Fair Value	3.8
Amount Carried in Balance Sheet	3.8
Other Bonds and Notes
SEC Schedule, 12-15, Insurance Companies, Summary of Investments, Other than Investments in Related Parties [Line Items]
Amortized Cost	4,046.8
Fair Value	3,690.8
Amount Carried in Balance Sheet	3,690.8
Redeemable Preferred Stocks
SEC Schedule, 12-15, Insurance Companies, Summary of Investments, Other than Investments in Related Parties [Line Items]
Amortized Cost	9.0
Fair Value	8.3
Amount Carried in Balance Sheet	8.3
Collateralized Loan Obligations
SEC Schedule, 12-15, Insurance Companies, Summary of Investments, Other than Investments in Related Parties [Line Items]
Amortized Cost	973.6
Fair Value	949.8
Amount Carried in Balance Sheet	949.8
Other Mortgage- and Asset-backed
SEC Schedule, 12-15, Insurance Companies, Summary of Investments, Other than Investments in Related Parties [Line Items]
Amortized Cost	362.0
Fair Value	315.8
Amount Carried in Balance Sheet	315.8
Equity Securities at Fair Value:
SEC Schedule, 12-15, Insurance Companies, Summary of Investments, Other than Investments in Related Parties [Line Items]
Amortized Cost	225.8
Fair Value	225.8
Amount Carried in Balance Sheet	225.8
Preferred Stocks
SEC Schedule, 12-15, Insurance Companies, Summary of Investments, Other than Investments in Related Parties [Line Items]
Amortized Cost	25.5
Fair Value	25.5
Amount Carried in Balance Sheet	25.5
Common Stock
SEC Schedule, 12-15, Insurance Companies, Summary of Investments, Other than Investments in Related Parties [Line Items]
Amortized Cost	1.2
Fair Value	1.2
Amount Carried in Balance Sheet	1.2
Other Equity Interests
SEC Schedule, 12-15, Insurance Companies, Summary of Investments, Other than Investments in Related Parties [Line Items]
Amortized Cost	199.1
Fair Value	199.1
Amount Carried in Balance Sheet	199.1
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	221.7
Amount Carried in Balance Sheet	221.7
Other Investments
SEC Schedule, 12-15, Insurance Companies, Summary of Investments, Other than Investments in Related Parties [Line Items]
Amortized Cost	241.9
Amount Carried in Balance Sheet	241.9
Short-term Investments
SEC Schedule, 12-15, Insurance Companies, Summary of Investments, Other than Investments in Related Parties [Line Items]
Amortized Cost	520.9
Amount Carried in Balance Sheet	520.9
Alternative Energy Partnership Investments
SEC Schedule, 12-15, Insurance Companies, Summary of Investments, Other than Investments in Related Parties [Line Items]
Amortized Cost	17.3
Amount Carried in Balance Sheet	17.3
Company-Owned Life Insurance
SEC Schedule, 12-15, Insurance Companies, Summary of Investments, Other than Investments in Related Parties [Line Items]
Amortized Cost	513.5
Amount Carried in Balance Sheet	513.5
Loans to Policyholders
SEC Schedule, 12-15, Insurance Companies, Summary of Investments, Other than Investments in Related Parties [Line Items]
Amortized Cost	281.2
Amount Carried in Balance Sheet	$ 281.2