Schedule I Summary of Investments-Other Than Investments in Related Parties (Details) $ in Millions	Dec. 31, 2025 USD ($)
SEC Schedule, 12-15, Insurance Companies, Summary of Investments, Other than Investments in Related Parties [Line Items]
Cost or amortized cost, net	$ 7,625.0
Balance Sheet	7,304.6
Fixed maturity securities
SEC Schedule, 12-15, Insurance Companies, Summary of Investments, Other than Investments in Related Parties [Line Items]
Cost or amortized cost, net	6,026.7
Fair Value	5,714.6
Balance Sheet	5,714.6
Equity securities
SEC Schedule, 12-15, Insurance Companies, Summary of Investments, Other than Investments in Related Parties [Line Items]
Cost or amortized cost, net	56.2
Fair Value	41.9
Balance Sheet	41.9
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 or amortized cost, net	1,099.6
Fair Value	1,014.8
Balance Sheet	1,014.8
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 or amortized cost, net	1,235.3
Fair Value	1,177.1
Balance Sheet	1,177.1
Foreign government bonds | Fixed maturity securities
SEC Schedule, 12-15, Insurance Companies, Summary of Investments, Other than Investments in Related Parties [Line Items]
Cost or amortized cost, net	10.6
Fair Value	10.0
Balance Sheet	10.0
Public utilities | Fixed maturity securities
SEC Schedule, 12-15, Insurance Companies, Summary of Investments, Other than Investments in Related Parties [Line Items]
Cost or amortized cost, net	176.3
Fair Value	161.0
Balance Sheet	161.0
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 or amortized cost, net	1,862.6
Fair Value	1,724.7
Balance Sheet	1,724.7
Asset-backed securities | Fixed maturity securities
SEC Schedule, 12-15, Insurance Companies, Summary of Investments, Other than Investments in Related Parties [Line Items]
Cost or amortized cost, net	1,159.7
Fair Value	1,160.8
Balance Sheet	1,160.8
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 or amortized cost, net	110.2
Fair Value	110.6
Balance Sheet	110.6
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 or amortized cost, net	346.5
Fair Value	331.0
Balance Sheet	331.0
Redeemable preferred stocks | Fixed maturity securities
SEC Schedule, 12-15, Insurance Companies, Summary of Investments, Other than Investments in Related Parties [Line Items]
Cost or amortized cost, net	25.9
Fair Value	24.6
Balance Sheet	24.6
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 or amortized cost, net	1.8
Fair Value	1.2
Balance Sheet	1.2
Non-redeemable preferred stocks | Equity securities
SEC Schedule, 12-15, Insurance Companies, Summary of Investments, Other than Investments in Related Parties [Line Items]
Cost or amortized cost, net	54.4
Fair Value	40.7
Balance Sheet	40.7
Limited partnership interests
SEC Schedule, 12-15, Insurance Companies, Summary of Investments, Other than Investments in Related Parties [Line Items]
Cost or amortized cost, net	1,100.6
Balance Sheet	1,100.6
Short-term investments
SEC Schedule, 12-15, Insurance Companies, Summary of Investments, Other than Investments in Related Parties [Line Items]
Cost or amortized cost, net	210.4
Balance Sheet	210.4
Policy loans
SEC Schedule, 12-15, Insurance Companies, Summary of Investments, Other than Investments in Related Parties [Line Items]
Cost or amortized cost, net	138.1
Balance Sheet	138.1
Derivatives
SEC Schedule, 12-15, Insurance Companies, Summary of Investments, Other than Investments in Related Parties [Line Items]
Cost or amortized cost, net	15.5
Fair Value	21.5
Balance Sheet	21.5
Mortgage loans
SEC Schedule, 12-15, Insurance Companies, Summary of Investments, Other than Investments in Related Parties [Line Items]
Cost or amortized cost, net	38.2
Balance Sheet	38.2
Other
SEC Schedule, 12-15, Insurance Companies, Summary of Investments, Other than Investments in Related Parties [Line Items]
Cost or amortized cost, net	39.3
Balance Sheet	$ 39.3