Fair Value Measurements - Fair Value Inputs (Details) - Fair Value, Measurements, Recurring - USD ($) $ in Millions | Sep. 30, 2024 | Dec. 31, 2023 |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Derivative Asset | | $ 14.7 |
Cash Flow Hedge Derivative Instrument Assets at Fair Value | $ 0.3 | |
Investments, Fair Value Disclosure | 7,100.6 | 7,122.4 |
Investments in Fixed Maturities | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Fixed maturities at fair value | 6,872 | 6,881.9 |
U.S. Government and Government Agencies and Authorities | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Fixed maturities at fair value | 521.9 | 511.5 |
States and Political Subdivisions | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Fixed maturities at fair value | 1,373.7 | 1,401.9 |
Foreign Governments | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Fixed maturities at fair value | 5.1 | 3.8 |
Bonds and Notes | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Fixed maturities at fair value | 3,753.3 | 3,690.8 |
Redeemable Preferred Stocks | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Fixed maturities at fair value | 10.3 | 8.3 |
Collateralized Loan Obligations | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Fixed maturities at fair value | 796.5 | 949.8 |
Other Mortgage- and Asset-backed | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Fixed maturities at fair value | 411.2 | 315.8 |
Equity Securities | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Equity securities | 228.3 | 225.8 |
Exchange Traded Funds | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Equity securities | 11 | 7.7 |
Limited Liability Companies and Limited Partnerships | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Equity securities | 189.2 | 191.4 |
Quoted Prices in Active Markets for Identical Assets (Level 1) | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Derivative Asset | | 0 |
Cash Flow Hedge Derivative Instrument Assets at Fair Value | 0 | |
Investments, Fair Value Disclosure | 99.9 | 107.3 |
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] | | |
Fixed maturities at fair value | 87.2 | 98.8 |
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] | | |
Fixed maturities at fair value | 87.2 | 98.8 |
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] | | |
Fixed maturities at fair value | 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] | | |
Fixed maturities at fair value | 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] | | |
Fixed maturities at fair value | 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] | | |
Fixed maturities at fair value | 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] | | |
Fixed maturities at fair value | 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] | | |
Fixed maturities at fair value | 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] | | |
Equity securities | 12.7 | 8.5 |
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] | | |
Equity securities | 11 | 7.7 |
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] | | |
Equity securities | 0 | 0 |
Significant Other Observable Inputs (Level 2) | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Derivative Asset | | 14.7 |
Cash Flow Hedge Derivative Instrument Assets at Fair Value | 0.3 | |
Investments, Fair Value Disclosure | 6,566.6 | 6,631.4 |
Significant Other Observable Inputs (Level 2) | Investments in Fixed Maturities | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Fixed maturities at fair value | 6,543.1 | 6,593.6 |
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] | | |
Fixed maturities at fair value | 434.7 | 412.7 |
Significant Other Observable Inputs (Level 2) | States and Political Subdivisions | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Fixed maturities at fair value | 1,372 | 1,401.8 |
Significant Other Observable Inputs (Level 2) | Foreign Governments | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Fixed maturities at fair value | 5.1 | 3.8 |
Significant Other Observable Inputs (Level 2) | Bonds and Notes | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Fixed maturities at fair value | 3,522.9 | 3,513.7 |
Significant Other Observable Inputs (Level 2) | Redeemable Preferred Stocks | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Fixed maturities at fair value | 6 | 1.2 |
Significant Other Observable Inputs (Level 2) | Collateralized Loan Obligations | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Fixed maturities at fair value | 796.5 | 949.8 |
Significant Other Observable Inputs (Level 2) | Other Mortgage- and Asset-backed | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Fixed maturities at fair value | 405.9 | 310.6 |
Significant Other Observable Inputs (Level 2) | Equity Securities | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Equity securities | 23.2 | 23.1 |
Significant Other Observable Inputs (Level 2) | Exchange Traded Funds | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Equity securities | 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] | | |
Equity securities | 0 | 0 |
Significant Unobservable Inputs (Level 3) | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Derivative Asset | | 0 |
Cash Flow Hedge Derivative Instrument Assets at Fair Value | 0 | |
Investments, Fair Value Disclosure | 244.9 | 192.3 |
Significant Unobservable Inputs (Level 3) | Investments in Fixed Maturities | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Fixed maturities at fair value | 241.7 | 189.5 |
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] | | |
Fixed maturities at fair value | 0 | 0 |
Significant Unobservable Inputs (Level 3) | States and Political Subdivisions | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Fixed maturities at fair value | 1.7 | 0.1 |
Significant Unobservable Inputs (Level 3) | Foreign Governments | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Fixed maturities at fair value | 0 | 0 |
Significant Unobservable Inputs (Level 3) | Bonds and Notes | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Fixed maturities at fair value | 230.4 | 177.1 |
Significant Unobservable Inputs (Level 3) | Redeemable Preferred Stocks | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Fixed maturities at fair value | 4.3 | 7.1 |
Significant Unobservable Inputs (Level 3) | Collateralized Loan Obligations | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Fixed maturities at fair value | 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] | | |
Fixed maturities at fair value | 5.3 | 5.2 |
Significant Unobservable Inputs (Level 3) | Equity Securities | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Equity securities | 3.2 | 2.8 |
Significant Unobservable Inputs (Level 3) | Exchange Traded Funds | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Equity securities | 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] | | |
Equity securities | 0 | 0 |
Measured at Net Asset Value | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Cash Flow Hedge Derivative Instrument Assets at Fair Value | 0 | |
Finance, Insurance and Real Estate | Preferred Stocks | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Equity securities | 16.3 | 15.6 |
Finance, Insurance and Real Estate | Common Stock | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Equity securities | 0.6 | 0.6 |
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] | | |
Equity securities | 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] | | |
Equity securities | 0.6 | 0.6 |
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] | | |
Equity securities | 16.3 | 15.6 |
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] | | |
Equity securities | 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] | | |
Equity securities | 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] | | |
Equity securities | 0 | 0 |
Other Industries | Preferred Stocks | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Equity securities | 9.4 | 9.9 |
Other Industries | Common Stock | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Equity securities | 1.8 | 0.6 |
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] | | |
Equity securities | 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] | | |
Equity securities | 1.1 | 0.2 |
Other Industries | Significant Other Observable Inputs (Level 2) | Preferred Stocks | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Equity securities | 6.9 | 7.5 |
Other Industries | Significant Other Observable Inputs (Level 2) | Common Stock | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Equity securities | 0 | 0 |
Other Industries | Significant Unobservable Inputs (Level 3) | Preferred Stocks | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Equity securities | 2.5 | 2.4 |
Other Industries | Significant Unobservable Inputs (Level 3) | Common Stock | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Equity securities | 0.7 | 0.4 |
Measured at Net Asset Value | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Derivative Asset | | 0 |
Investments, Fair Value Disclosure | 189.2 | 191.4 |
Measured at Net Asset Value | Investments in Fixed Maturities | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Fixed maturities at fair value | 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] | | |
Fixed maturities at fair value | 0 | 0 |
Measured at Net Asset Value | States and Political Subdivisions | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Fixed maturities at fair value | 0 | 0 |
Measured at Net Asset Value | Foreign Governments | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Fixed maturities at fair value | 0 | 0 |
Measured at Net Asset Value | Bonds and Notes | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Fixed maturities at fair value | 0 | 0 |
Measured at Net Asset Value | Redeemable Preferred Stocks | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Fixed maturities at fair value | 0 | 0 |
Measured at Net Asset Value | Collateralized Loan Obligations | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Fixed maturities at fair value | 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] | | |
Fixed maturities at fair value | 0 | 0 |
Measured at Net Asset Value | Equity Securities | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Equity securities | 189.2 | 191.4 |
Measured at Net Asset Value | Exchange Traded Funds | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Equity securities | 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] | | |
Equity securities | 189.2 | 191.4 |
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] | | |
Equity securities | 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] | | |
Equity securities | 0 | 0 |
Measured at Net Asset Value | Other Industries | Preferred Stocks | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Equity securities | 0 | 0 |
Measured at Net Asset Value | Other Industries | Common Stock | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Equity securities | $ 0 | $ 0 |