Schedule I Summary of Investments-Other Than Investments in Related Parties (Details) $ in Millions	Dec. 31, 2022 USD ($)
SEC Schedule, 12-15, Insurance Companies, Summary of Investments, Other than Investments in Related Parties [Line Items]
Cost	$ 7,162.7
Balance Sheet	6,587.6
Fixed maturity securities
SEC Schedule, 12-15, Insurance Companies, Summary of Investments, Other than Investments in Related Parties [Line Items]
Cost	5,756.9
Cost or amortized cost, net	5,185.0
Balance Sheet	5,185.0
Equity securities
SEC Schedule, 12-15, Insurance Companies, Summary of Investments, Other than Investments in Related Parties [Line Items]
Cost	99.6
Cost or amortized cost, net	99.6
Balance Sheet	99.6
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	830.4
Cost or amortized cost, net	715.4
Balance Sheet	715.4
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	1,380.9
Cost or amortized cost, net	1,269.7
Balance Sheet	1,269.7
Foreign government bonds | Fixed maturity securities
SEC Schedule, 12-15, Insurance Companies, Summary of Investments, Other than Investments in Related Parties [Line Items]
Cost	35.2
Cost or amortized cost, net	33.6
Balance Sheet	33.6
Public utilities | Fixed maturity securities
SEC Schedule, 12-15, Insurance Companies, Summary of Investments, Other than Investments in Related Parties [Line Items]
Cost	101.8
Cost or amortized cost, net	86.1
Balance Sheet	86.1
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	2,022.2
Cost or amortized cost, net	1,782.4
Balance Sheet	1,782.4
Asset-backed securities | Fixed maturity securities
SEC Schedule, 12-15, Insurance Companies, Summary of Investments, Other than Investments in Related Parties [Line Items]
Cost	1,006.7
Cost or amortized cost, net	954.8
Balance Sheet	954.8
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	12.9
Cost or amortized cost, net	11.7
Balance Sheet	11.7
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	329.6
Cost or amortized cost, net	298.1
Balance Sheet	298.1
Redeemable preferred stocks | Fixed maturity securities
SEC Schedule, 12-15, Insurance Companies, Summary of Investments, Other than Investments in Related Parties [Line Items]
Cost	37.2
Cost or amortized cost, net	33.2
Balance Sheet	33.2
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	0.8
Cost or amortized cost, net	0.8
Balance Sheet	0.8
Non-redeemable preferred stocks | Equity securities
SEC Schedule, 12-15, Insurance Companies, Summary of Investments, Other than Investments in Related Parties [Line Items]
Cost	81.8
Cost or amortized cost, net	81.8
Balance Sheet	81.8
Closed-end fund | Equity securities
SEC Schedule, 12-15, Insurance Companies, Summary of Investments, Other than Investments in Related Parties [Line Items]
Cost	16.7
Cost or amortized cost, net	16.7
Balance Sheet	16.7
Limited partnership interests
SEC Schedule, 12-15, Insurance Companies, Summary of Investments, Other than Investments in Related Parties [Line Items]
Cost	983.7
Balance Sheet	983.7
Short-term Investments
SEC Schedule, 12-15, Insurance Companies, Summary of Investments, Other than Investments in Related Parties [Line Items]
Cost	109.4
Balance Sheet	109.4
Policy loans
SEC Schedule, 12-15, Insurance Companies, Summary of Investments, Other than Investments in Related Parties [Line Items]
Cost	139.3
Balance Sheet	139.3
Derivatives
SEC Schedule, 12-15, Insurance Companies, Summary of Investments, Other than Investments in Related Parties [Line Items]
Cost	10.0
Cost or amortized cost, net	6.8
Balance Sheet	6.8
Mortgage loans
SEC Schedule, 12-15, Insurance Companies, Summary of Investments, Other than Investments in Related Parties [Line Items]
Cost	32.0
Balance Sheet	32.0
Other
SEC Schedule, 12-15, Insurance Companies, Summary of Investments, Other than Investments in Related Parties [Line Items]
Cost	31.8
Balance Sheet	$ 31.8