Schedule I Summary of 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]
Cost	$ 7,241.6
Balance Sheet	6,830.5
Fixed maturity securities
SEC Schedule, 12-15, Insurance Companies, Summary of Investments, Other than Investments in Related Parties [Line Items]
Cost	5,652.9
Cost or amortized cost, net	5,235.3
Balance Sheet	5,235.3
Equity securities
SEC Schedule, 12-15, Insurance Companies, Summary of Investments, Other than Investments in Related Parties [Line Items]
Cost	86.2
Cost or amortized cost, net	86.2
Balance Sheet	86.2
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	925.0
Cost or amortized cost, net	819.2
Balance Sheet	819.2
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	1,333.4
Cost or amortized cost, net	1,270.1
Balance Sheet	1,270.1
Foreign government bonds | Fixed maturity securities
SEC Schedule, 12-15, Insurance Companies, Summary of Investments, Other than Investments in Related Parties [Line Items]
Cost	23.1
Cost or amortized cost, net	22.1
Balance Sheet	22.1
Public utilities | Fixed maturity securities
SEC Schedule, 12-15, Insurance Companies, Summary of Investments, Other than Investments in Related Parties [Line Items]
Cost	93.8
Cost or amortized cost, net	80.9
Balance Sheet	80.9
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	1,850.0
Cost or amortized cost, net	1,665.5
Balance Sheet	1,665.5
Asset-backed securities | Fixed maturity securities
SEC Schedule, 12-15, Insurance Companies, Summary of Investments, Other than Investments in Related Parties [Line Items]
Cost	1,044.7
Cost or amortized cost, net	1,020.5
Balance Sheet	1,020.5
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	13.9
Cost or amortized cost, net	12.9
Balance Sheet	12.9
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	342.9
Cost or amortized cost, net	317.8
Balance Sheet	317.8
Redeemable preferred stocks | Fixed maturity securities
SEC Schedule, 12-15, Insurance Companies, Summary of Investments, Other than Investments in Related Parties [Line Items]
Cost	26.1
Cost or amortized cost, net	26.3
Balance Sheet	26.3
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	0.3
Cost or amortized cost, net	0.3
Balance Sheet	0.3
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	1.1
Cost or amortized cost, net	1.1
Balance Sheet	1.1
Non-redeemable preferred stocks | Equity securities
SEC Schedule, 12-15, Insurance Companies, Summary of Investments, Other than Investments in Related Parties [Line Items]
Cost	68.3
Cost or amortized cost, net	68.3
Balance Sheet	68.3
Closed-end fund | Equity securities
SEC Schedule, 12-15, Insurance Companies, Summary of Investments, Other than Investments in Related Parties [Line Items]
Cost	16.5
Cost or amortized cost, net	16.5
Balance Sheet	16.5
Limited partnership interests
SEC Schedule, 12-15, Insurance Companies, Summary of Investments, Other than Investments in Related Parties [Line Items]
Cost	1,138.8
Balance Sheet	1,138.8
Short-term Investments
SEC Schedule, 12-15, Insurance Companies, Summary of Investments, Other than Investments in Related Parties [Line Items]
Cost	132.9
Balance Sheet	132.9
Policy loans
SEC Schedule, 12-15, Insurance Companies, Summary of Investments, Other than Investments in Related Parties [Line Items]
Cost	141.4
Balance Sheet	141.4
Derivatives
SEC Schedule, 12-15, Insurance Companies, Summary of Investments, Other than Investments in Related Parties [Line Items]
Cost	12.5
Cost or amortized cost, net	19.0
Balance Sheet	19.0
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	33.7
Balance Sheet	$ 33.7