Schedule 1 - Investments Other Than Investments in Related Parties (Details) $ in Millions	Dec. 31, 2024 USD ($)
SEC Schedule, 12-15, Insurance Companies, Summary of Investments, Other than Investments in Related Parties [Line Items]
Amortized Cost	$ 9,773.9
Amount Carried in Balance Sheet	8,888.5
Fixed Maturities:
SEC Schedule, 12-15, Insurance Companies, Summary of Investments, Other than Investments in Related Parties [Line Items]
Amortized Cost	7,295.0
Fair Value	6,409.6
Amount Carried in Balance Sheet	6,409.6
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	588.6
Fair Value	486.8
Amount Carried in Balance Sheet	486.8
States and Political Subdivisions
SEC Schedule, 12-15, Insurance Companies, Summary of Investments, Other than Investments in Related Parties [Line Items]
Amortized Cost	1,457.3
Fair Value	1,233.2
Amount Carried in Balance Sheet	1,233.2
Foreign Governments
SEC Schedule, 12-15, Insurance Companies, Summary of Investments, Other than Investments in Related Parties [Line Items]
Amortized Cost	6.5
Fair Value	6.6
Amount Carried in Balance Sheet	6.6
Other Bonds and Notes
SEC Schedule, 12-15, Insurance Companies, Summary of Investments, Other than Investments in Related Parties [Line Items]
Amortized Cost	4,038.3
Fair Value	3,519.6
Amount Carried in Balance Sheet	3,519.6
Redeemable Preferred Stocks
SEC Schedule, 12-15, Insurance Companies, Summary of Investments, Other than Investments in Related Parties [Line Items]
Amortized Cost	9.8
Fair Value	8.9
Amount Carried in Balance Sheet	8.9
Collateralized Loan Obligations
SEC Schedule, 12-15, Insurance Companies, Summary of Investments, Other than Investments in Related Parties [Line Items]
Amortized Cost	747.8
Fair Value	741.5
Amount Carried in Balance Sheet	741.5
Other Mortgage- and Asset-backed
SEC Schedule, 12-15, Insurance Companies, Summary of Investments, Other than Investments in Related Parties [Line Items]
Amortized Cost	446.7
Fair Value	413.0
Amount Carried in Balance Sheet	413.0
Equity Securities at Fair Value:
SEC Schedule, 12-15, Insurance Companies, Summary of Investments, Other than Investments in Related Parties [Line Items]
Amortized Cost	218.5
Fair Value	218.5
Amount Carried in Balance Sheet	218.5
Preferred Stocks
SEC Schedule, 12-15, Insurance Companies, Summary of Investments, Other than Investments in Related Parties [Line Items]
Amortized Cost	22.6
Fair Value	22.6
Amount Carried in Balance Sheet	22.6
Common Stock
SEC Schedule, 12-15, Insurance Companies, Summary of Investments, Other than Investments in Related Parties [Line Items]
Amortized Cost	1.4
Fair Value	1.4
Amount Carried in Balance Sheet	1.4
Other Equity Interests
SEC Schedule, 12-15, Insurance Companies, Summary of Investments, Other than Investments in Related Parties [Line Items]
Amortized Cost	194.5
Fair Value	194.5
Amount Carried in Balance Sheet	194.5
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	186.3
Amount Carried in Balance Sheet	186.3
Alternative Energy Partnership Investments
SEC Schedule, 12-15, Insurance Companies, Summary of Investments, Other than Investments in Related Parties [Line Items]
Amortized Cost	17.6
Amount Carried in Balance Sheet	17.6
Short-term Investments
SEC Schedule, 12-15, Insurance Companies, Summary of Investments, Other than Investments in Related Parties [Line Items]
Amortized Cost	1,037.1
Amount Carried in Balance Sheet	1,037.1
Company-Owned Life Insurance
SEC Schedule, 12-15, Insurance Companies, Summary of Investments, Other than Investments in Related Parties [Line Items]
Amortized Cost	539.2
Amount Carried in Balance Sheet	539.2
Loans to Policyholders
SEC Schedule, 12-15, Insurance Companies, Summary of Investments, Other than Investments in Related Parties [Line Items]
Amortized Cost	280.7
Amount Carried in Balance Sheet	280.7
Other Investments
SEC Schedule, 12-15, Insurance Companies, Summary of Investments, Other than Investments in Related Parties [Line Items]
Amortized Cost	199.5
Amount Carried in Balance Sheet	$ 199.5