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Fair Value Measurements - Fair Value Inputs (Details) - Fair Value, Measurements, Recurring - USD ($)
$ in Millions
Sep. 30, 2025
Dec. 31, 2024
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Derivative Instrument Classified as Cash Flow Hedge $ 2.7  
Total Assets 6,925.0 $ 6,628.1
Total Liabilities   (3.7)
Investments in Fixed Maturities    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Total Investments in Fixed Maturities 6,620.3 6,409.6
U.S. Government and Government Agencies and Authorities    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Total Investments in Fixed Maturities 591.9 486.8
States and Political Subdivisions    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Total Investments in Fixed Maturities 1,243.2 1,233.2
Foreign Governments    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Total Investments in Fixed Maturities 12.0 6.6
Bonds and Notes    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Total Investments in Fixed Maturities 3,574.2 3,519.6
Redeemable Preferred Stocks    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Total Investments in Fixed Maturities 10.3 8.9
Collateralized Loan Obligations    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Total Investments in Fixed Maturities 817.9 741.5
Other Mortgage- and Asset-backed    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Total Investments in Fixed Maturities 370.8 413.0
Equity Securities    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Total Investments in Equity Securities at Fair Value 302.0 218.5
Exchange Traded Funds    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Total Investments in Equity Securities at Fair Value 76.6 10.9
Limited Liability Companies and Limited Partnerships    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Total Investments in Equity Securities at Fair Value 204.0 183.6
Designated as Hedging Instrument    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Derivative Instruments Designated as Cash Flow Hedges   (3.7)
Quoted Prices in Active Markets for Identical Assets (Level 1)    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Derivative Instrument Classified as Cash Flow Hedge 0.0  
Total Assets 180.7 98.1
Total Liabilities   0.0
Quoted Prices in Active Markets for Identical Assets (Level 1) | Investments in Fixed Maturities    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Total Investments in Fixed Maturities 103.9 86.8
Quoted Prices in Active Markets for Identical Assets (Level 1) | U.S. Government and Government Agencies and Authorities    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Total Investments in Fixed Maturities 103.9 86.8
Quoted Prices in Active Markets for Identical Assets (Level 1) | States and Political Subdivisions    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Total Investments in Fixed Maturities 0.0 0.0
Quoted Prices in Active Markets for Identical Assets (Level 1) | Foreign Governments    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Total Investments in Fixed Maturities 0.0 0.0
Quoted Prices in Active Markets for Identical Assets (Level 1) | Bonds and Notes    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Total Investments in Fixed Maturities 0.0 0.0
Quoted Prices in Active Markets for Identical Assets (Level 1) | Redeemable Preferred Stocks    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Total Investments in Fixed Maturities 0.0 0.0
Quoted Prices in Active Markets for Identical Assets (Level 1) | Collateralized Loan Obligations    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Total Investments in Fixed Maturities 0.0 0.0
Quoted Prices in Active Markets for Identical Assets (Level 1) | Other Mortgage- and Asset-backed    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Total Investments in Fixed Maturities 0.0 0.0
Quoted Prices in Active Markets for Identical Assets (Level 1) | Equity Securities    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Total Investments in Equity Securities at Fair Value 76.8 11.3
Quoted Prices in Active Markets for Identical Assets (Level 1) | Exchange Traded Funds    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Total Investments in Equity Securities at Fair Value 76.6 10.9
Quoted Prices in Active Markets for Identical Assets (Level 1) | Limited Liability Companies and Limited Partnerships    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Total Investments in Equity Securities at Fair Value 0.0 0.0
Quoted Prices in Active Markets for Identical Assets (Level 1) | Designated as Hedging Instrument    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Derivative Instruments Designated as Cash Flow Hedges   0.0
Significant Other Observable Inputs (Level 2)    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Derivative Instrument Classified as Cash Flow Hedge 2.7  
Total Assets 6,206.3 6,137.4
Total Liabilities   (3.7)
Significant Other Observable Inputs (Level 2) | Investments in Fixed Maturities    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Total Investments in Fixed Maturities 6,186.4 6,117.6
Significant Other Observable Inputs (Level 2) | U.S. Government and Government Agencies and Authorities    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Total Investments in Fixed Maturities 488.0 400.0
Significant Other Observable Inputs (Level 2) | States and Political Subdivisions    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Total Investments in Fixed Maturities 1,241.4 1,231.4
Significant Other Observable Inputs (Level 2) | Foreign Governments    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Total Investments in Fixed Maturities 9.5 6.6
Significant Other Observable Inputs (Level 2) | Bonds and Notes    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Total Investments in Fixed Maturities 3,293.5 3,325.4
Significant Other Observable Inputs (Level 2) | Redeemable Preferred Stocks    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Total Investments in Fixed Maturities 5.9 4.7
Significant Other Observable Inputs (Level 2) | Collateralized Loan Obligations    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Total Investments in Fixed Maturities 812.2 741.5
Significant Other Observable Inputs (Level 2) | Other Mortgage- and Asset-backed    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Total Investments in Fixed Maturities 335.9 408.0
Significant Other Observable Inputs (Level 2) | Equity Securities    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Total Investments in Equity Securities at Fair Value 17.2 19.8
Significant Other Observable Inputs (Level 2) | Exchange Traded Funds    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Total Investments in Equity Securities at Fair Value 0.0 0.0
Significant Other Observable Inputs (Level 2) | Limited Liability Companies and Limited Partnerships    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Total Investments in Equity Securities at Fair Value 0.0 0.0
Significant Other Observable Inputs (Level 2) | Designated as Hedging Instrument    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Derivative Instruments Designated as Cash Flow Hedges   (3.7)
Significant Unobservable Inputs (Level 3)    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Derivative Instrument Classified as Cash Flow Hedge 0.0  
Total Assets 334.0 209.0
Total Liabilities   0.0
Significant Unobservable Inputs (Level 3) | Investments in Fixed Maturities    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Total Investments in Fixed Maturities 330.0 205.2
Significant Unobservable Inputs (Level 3) | U.S. Government and Government Agencies and Authorities    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Total Investments in Fixed Maturities 0.0 0.0
Significant Unobservable Inputs (Level 3) | States and Political Subdivisions    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Total Investments in Fixed Maturities 1.8 1.8
Significant Unobservable Inputs (Level 3) | Foreign Governments    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Total Investments in Fixed Maturities 2.5 0.0
Significant Unobservable Inputs (Level 3) | Bonds and Notes    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Total Investments in Fixed Maturities 280.7 194.2
Significant Unobservable Inputs (Level 3) | Redeemable Preferred Stocks    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Total Investments in Fixed Maturities 4.4 4.2
Significant Unobservable Inputs (Level 3) | Collateralized Loan Obligations    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Total Investments in Fixed Maturities 5.7 0.0
Significant Unobservable Inputs (Level 3) | Other Mortgage- and Asset-backed    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Total Investments in Fixed Maturities 34.9 5.0
Significant Unobservable Inputs (Level 3) | Equity Securities    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Total Investments in Equity Securities at Fair Value 4.0 3.8
Significant Unobservable Inputs (Level 3) | Exchange Traded Funds    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Total Investments in Equity Securities at Fair Value 0.0 0.0
Significant Unobservable Inputs (Level 3) | Limited Liability Companies and Limited Partnerships    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Total Investments in Equity Securities at Fair Value 0.0 0.0
Significant Unobservable Inputs (Level 3) | Designated as Hedging Instrument    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Derivative Instruments Designated as Cash Flow Hedges   0.0
Finance, Insurance and Real Estate | Preferred Stocks    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Total Investments in Equity Securities at Fair Value 14.2 13.1
Finance, Insurance and Real Estate | Common Stock    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Total Investments in Equity Securities at Fair Value 0.0 0.3
Finance, Insurance and Real Estate | Quoted Prices in Active Markets for Identical Assets (Level 1) | Preferred Stocks    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Total Investments in Equity Securities at Fair Value 0.0 0.0
Finance, Insurance and Real Estate | Quoted Prices in Active Markets for Identical Assets (Level 1) | Common Stock    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Total Investments in Equity Securities at Fair Value 0.0 0.3
Finance, Insurance and Real Estate | Significant Other Observable Inputs (Level 2) | Preferred Stocks    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Total Investments in Equity Securities at Fair Value 11.6 13.1
Finance, Insurance and Real Estate | Significant Other Observable Inputs (Level 2) | Common Stock    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Total Investments in Equity Securities at Fair Value 0.0 0.0
Finance, Insurance and Real Estate | Significant Unobservable Inputs (Level 3) | Preferred Stocks    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Total Investments in Equity Securities at Fair Value 2.6 0.0
Finance, Insurance and Real Estate | Significant Unobservable Inputs (Level 3) | Common Stock    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Total Investments in Equity Securities at Fair Value 0.0 0.0
Other Industries | Preferred Stocks    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Total Investments in Equity Securities at Fair Value 5.6 9.5
Other Industries | Common Stock    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Total Investments in Equity Securities at Fair Value 1.6 1.1
Other Industries | Quoted Prices in Active Markets for Identical Assets (Level 1) | Preferred Stocks    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Total Investments in Equity Securities at Fair Value 0.0 0.0
Other Industries | Quoted Prices in Active Markets for Identical Assets (Level 1) | Common Stock    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Total Investments in Equity Securities at Fair Value 0.2 0.1
Other Industries | Significant Other Observable Inputs (Level 2) | Preferred Stocks    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Total Investments in Equity Securities at Fair Value 5.6 6.7
Other Industries | Significant Other Observable Inputs (Level 2) | Common Stock    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Total Investments in Equity Securities at Fair Value 0.0 0.0
Other Industries | Significant Unobservable Inputs (Level 3) | Preferred Stocks    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Total Investments in Equity Securities at Fair Value 0.0 2.8
Other Industries | Significant Unobservable Inputs (Level 3) | Common Stock    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Total Investments in Equity Securities at Fair Value 1.4 1.0
Measured at Net Asset Value    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Derivative Instrument Classified as Cash Flow Hedge 0.0  
Total Assets 204.0 183.6
Total Liabilities   0.0
Measured at Net Asset Value | Investments in Fixed Maturities    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Total Investments in Fixed Maturities 0.0 0.0
Measured at Net Asset Value | U.S. Government and Government Agencies and Authorities    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Total Investments in Fixed Maturities 0.0 0.0
Measured at Net Asset Value | States and Political Subdivisions    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Total Investments in Fixed Maturities 0.0 0.0
Measured at Net Asset Value | Foreign Governments    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Total Investments in Fixed Maturities 0.0 0.0
Measured at Net Asset Value | Bonds and Notes    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Total Investments in Fixed Maturities 0.0 0.0
Measured at Net Asset Value | Redeemable Preferred Stocks    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Total Investments in Fixed Maturities 0.0 0.0
Measured at Net Asset Value | Collateralized Loan Obligations    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Total Investments in Fixed Maturities 0.0 0.0
Measured at Net Asset Value | Other Mortgage- and Asset-backed    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Total Investments in Fixed Maturities 0.0 0.0
Measured at Net Asset Value | Equity Securities    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Total Investments in Equity Securities at Fair Value 204.0 183.6
Measured at Net Asset Value | Exchange Traded Funds    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Total Investments in Equity Securities at Fair Value 0.0 0.0
Measured at Net Asset Value | Limited Liability Companies and Limited Partnerships    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Total Investments in Equity Securities at Fair Value 204.0 183.6
Measured at Net Asset Value | Designated as Hedging Instrument    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Derivative Instruments Designated as Cash Flow Hedges   0.0
Measured at Net Asset Value | Finance, Insurance and Real Estate | Preferred Stocks    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Total Investments in Equity Securities at Fair Value 0.0 0.0
Measured at Net Asset Value | Finance, Insurance and Real Estate | Common Stock    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Total Investments in Equity Securities at Fair Value 0.0 0.0
Measured at Net Asset Value | Other Industries | Preferred Stocks    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Total Investments in Equity Securities at Fair Value 0.0 0.0
Measured at Net Asset Value | Other Industries | Common Stock    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Total Investments in Equity Securities at Fair Value $ 0.0 $ 0.0