v3.26.1
Fair Value Measurements - Valuation of Assets Measured at Fair Value (Details) - Fair Value, Measurements, Recurring - USD ($)
$ in Millions
Jun. 30, 2026
Dec. 31, 2025
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Total Assets $ 7,030.7 $ 7,049.7
Investments in Fixed Maturities    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Total Investments in Fixed Maturities 6,721.1 6,743.3
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 641.8 622.4
States and Political Subdivisions    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Total Investments in Fixed Maturities 1,233.3 1,253.2
Foreign Governments    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Total Investments in Fixed Maturities 8.5 11.0
Bonds and Notes    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Total Investments in Fixed Maturities 3,710.3 3,650.3
Redeemable Preferred Stocks    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Total Investments in Fixed Maturities 9.8 10.2
Collateralized Loan Obligations    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Total Investments in Fixed Maturities 768.8 843.0
Other Mortgage- and Asset-backed    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Total Investments in Fixed Maturities 348.6 353.2
Equity Securities    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Total Investments in Equity Securities 309.6 306.4
Exchange Traded Funds    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Total Investments in Equity Securities 11.6 11.9
Limited Liability Companies and Limited Partnerships    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Total Investments in Equity Securities 221.5 210.3
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 14.1 14.2
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 54.2 63.2
Other Industries | Preferred Stocks    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Total Investments in Equity Securities 5.3 5.0
Other Industries | Common Stock    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Total Investments in Equity Securities 2.9 1.8
Measured at NAV    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Total Assets 221.5 210.3
Measured at NAV | 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 NAV | 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 NAV | 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 NAV | 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 NAV | 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 NAV | 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 NAV | 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 NAV | 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 NAV | Equity Securities    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Total Investments in Equity Securities 221.5 210.3
Measured at NAV | Exchange Traded Funds    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Total Investments in Equity Securities 0.0 0.0
Measured at NAV | Limited Liability Companies and Limited Partnerships    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Total Investments in Equity Securities 221.5 210.3
Measured at NAV | 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 0.0 0.0
Measured at NAV | 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 0.0 0.0
Measured at NAV | Other Industries | Preferred Stocks    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Total Investments in Equity Securities 0.0 0.0
Measured at NAV | Other Industries | Common Stock    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Total Investments in Equity Securities 0.0 0.0
Quoted Prices in Active Markets for Identical Assets (Level 1)    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Total Assets 174.1 191.2
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 108.0 115.9
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 108.0 115.9
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 66.1 75.3
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 11.6 11.9
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 0.0 0.0
Quoted Prices in Active Markets for Identical Assets (Level 1) | 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 0.0 0.0
Quoted Prices in Active Markets for Identical Assets (Level 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 54.2 63.2
Quoted Prices in Active Markets for Identical Assets (Level 1) | Other Industries | Preferred Stocks    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Total Investments in Equity Securities 0.0 0.0
Quoted Prices in Active Markets for Identical Assets (Level 1) | Other Industries | Common Stock    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Total Investments in Equity Securities 0.3 0.2
Significant Other Observable Inputs (Level 2)    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Total Assets 6,149.4 6,304.6
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 6,132.9 6,288.0
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 533.8 506.5
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 1,233.1 1,251.5
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 8.5 11.0
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 3,284.3 3,357.3
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 5.8 5.8
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 764.8 830.6
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 302.6 325.3
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 16.5 16.6
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 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 0.0 0.0
Significant Other Observable Inputs (Level 2) | 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 11.5 11.6
Significant Other Observable Inputs (Level 2) | 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 0.0 0.0
Significant Other Observable Inputs (Level 2) | Other Industries | Preferred Stocks    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Total Investments in Equity Securities 5.0 5.0
Significant Other Observable Inputs (Level 2) | Other Industries | Common Stock    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Total Investments in Equity Securities 0.0 0.0
Significant Unobservable Inputs (Level 3)    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Total Assets 485.7 343.6
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 480.2 339.4
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 0.2 1.7
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 0.0 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 426.0 293.0
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 4.0 4.4
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 4.0 12.4
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 46.0 27.9
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 5.5 4.2
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 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 0.0 0.0
Significant Unobservable Inputs (Level 3) | 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 2.6 2.6
Significant Unobservable Inputs (Level 3) | 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 0.0 0.0
Significant Unobservable Inputs (Level 3) | Other Industries | Preferred Stocks    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Total Investments in Equity Securities 0.3 0.0
Significant Unobservable Inputs (Level 3) | Other Industries | Common Stock    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Total Investments in Equity Securities $ 2.6 $ 1.6