Schedule I Summary of 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]
Cost	$ 7,379.1
Balance Sheet	6,916.4
Fixed maturity securities
SEC Schedule, 12-15, Insurance Companies, Summary of Investments, Other than Investments in Related Parties [Line Items]
Cost	5,842.5
Cost or amortized cost, net	5,387.9
Balance Sheet	5,387.9
Equity securities
SEC Schedule, 12-15, Insurance Companies, Summary of Investments, Other than Investments in Related Parties [Line Items]
Cost	78.8
Cost or amortized cost, net	66.5
Balance Sheet	66.5
U.S. Government and federally sponsored agency obligations | Fixed maturity securities
SEC Schedule, 12-15, Insurance Companies, Summary of Investments, Other than Investments in Related Parties [Line Items]
Cost	1,029.0
Cost or amortized cost, net	909.3
Balance Sheet	909.3
States, municipalities and political subdivisions | Fixed maturity securities
SEC Schedule, 12-15, Insurance Companies, Summary of Investments, Other than Investments in Related Parties [Line Items]
Cost	1,239.1
Cost or amortized cost, net	1,150.8
Balance Sheet	1,150.8
Foreign government bonds | Fixed maturity securities
SEC Schedule, 12-15, Insurance Companies, Summary of Investments, Other than Investments in Related Parties [Line Items]
Cost	14.1
Cost or amortized cost, net	13.1
Balance Sheet	13.1
Public utilities | Fixed maturity securities
SEC Schedule, 12-15, Insurance Companies, Summary of Investments, Other than Investments in Related Parties [Line Items]
Cost	106.2
Cost or amortized cost, net	91.1
Balance Sheet	91.1
All other corporate bonds | Fixed maturity securities
SEC Schedule, 12-15, Insurance Companies, Summary of Investments, Other than Investments in Related Parties [Line Items]
Cost	1,857.3
Cost or amortized cost, net	1,660.3
Balance Sheet	1,660.3
Asset-backed securities | Fixed maturity securities
SEC Schedule, 12-15, Insurance Companies, Summary of Investments, Other than Investments in Related Parties [Line Items]
Cost	1,177.0
Cost or amortized cost, net	1,170.9
Balance Sheet	1,170.9
Residential mortgage-backed securities (non-agency) | Fixed maturity securities
SEC Schedule, 12-15, Insurance Companies, Summary of Investments, Other than Investments in Related Parties [Line Items]
Cost	69.1
Cost or amortized cost, net	68.5
Balance Sheet	68.5
Commercial mortgage-backed securities | Fixed maturity securities
SEC Schedule, 12-15, Insurance Companies, Summary of Investments, Other than Investments in Related Parties [Line Items]
Cost	319.0
Cost or amortized cost, net	292.9
Balance Sheet	292.9
Redeemable preferred stocks | Fixed maturity securities
SEC Schedule, 12-15, Insurance Companies, Summary of Investments, Other than Investments in Related Parties [Line Items]
Cost	31.7
Cost or amortized cost, net	31.0
Balance Sheet	31.0
Industrial, miscellaneous and all other | Equity securities
SEC Schedule, 12-15, Insurance Companies, Summary of Investments, Other than Investments in Related Parties [Line Items]
Cost	0.0
Cost or amortized cost, net	0.5
Balance Sheet	0.5
Banking & finance and insurance companies | Equity securities
SEC Schedule, 12-15, Insurance Companies, Summary of Investments, Other than Investments in Related Parties [Line Items]
Cost	1.8
Cost or amortized cost, net	1.5
Balance Sheet	1.5
Non-redeemable preferred stocks | Equity securities
SEC Schedule, 12-15, Insurance Companies, Summary of Investments, Other than Investments in Related Parties [Line Items]
Cost	77.0
Cost or amortized cost, net	64.5
Balance Sheet	64.5
Limited partnership interests(1)
SEC Schedule, 12-15, Insurance Companies, Summary of Investments, Other than Investments in Related Parties [Line Items]
Cost	1,121.3
Balance Sheet	1,121.3
Short-term Investments
SEC Schedule, 12-15, Insurance Companies, Summary of Investments, Other than Investments in Related Parties [Line Items]
Cost	101.2
Balance Sheet	101.2
Policy loans
SEC Schedule, 12-15, Insurance Companies, Summary of Investments, Other than Investments in Related Parties [Line Items]
Cost	140.8
Balance Sheet	140.8
Derivatives
SEC Schedule, 12-15, Insurance Companies, Summary of Investments, Other than Investments in Related Parties [Line Items]
Cost	14.3
Cost or amortized cost, net	18.5
Balance Sheet	18.5
Mortgage loans
SEC Schedule, 12-15, Insurance Companies, Summary of Investments, Other than Investments in Related Parties [Line Items]
Cost	43.2
Balance Sheet	43.2
Other
SEC Schedule, 12-15, Insurance Companies, Summary of Investments, Other than Investments in Related Parties [Line Items]
Cost	37.0
Balance Sheet	$ 37.0