Fair Value Measurements - Fair Value Inputs (Details) - Fair Value, Measurements, Recurring - USD ($) $ in Millions | Jun. 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 | $ 0.8 | |
Investments, Fair Value Disclosure | 6,902.1 | 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 | 6,674.7 | 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 | 475.5 | 511.5 |
US States and Political Subdivisions Debt Securities [Member] | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Fixed maturities at fair value | 1,339.7 | 1,401.9 |
Foreign Governments | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Fixed maturities at fair value | 4.6 | 3.8 |
Corporate Debt Securities [Member] | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Fixed maturities at fair value | 3,586.6 | 3,690.8 |
Redeemable Preferred Stocks | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Fixed maturities at fair value | 9.9 | 8.3 |
Collateralized Loan Obligations | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Fixed maturities at fair value | 885.9 | 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 | 372.5 | 315.8 |
Equity Securities | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Equity securities | 226.6 | 225.8 |
Exchange Traded Funds [Member] | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Equity securities | 9.7 | 7.7 |
Partnership Interest [Member] | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Equity securities | 189.3 | 191.4 |
Fair Value, Inputs, Level 1 [Member] | | |
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 | 95.5 | 107.3 |
Fair Value, Inputs, Level 1 [Member] | Investments in Fixed Maturities | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Fixed maturities at fair value | 84.2 | 98.8 |
Fair Value, Inputs, Level 1 [Member] | 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 | 84.2 | 98.8 |
Fair Value, Inputs, Level 1 [Member] | US States and Political Subdivisions Debt Securities [Member] | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Fixed maturities at fair value | 0 | 0 |
Fair Value, Inputs, Level 1 [Member] | Foreign Governments | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Fixed maturities at fair value | 0 | 0 |
Fair Value, Inputs, Level 1 [Member] | Corporate Debt Securities [Member] | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Fixed maturities at fair value | 0 | 0 |
Fair Value, Inputs, Level 1 [Member] | Redeemable Preferred Stocks | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Fixed maturities at fair value | 0 | 0 |
Fair Value, Inputs, Level 1 [Member] | Collateralized Loan Obligations | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Fixed maturities at fair value | 0 | 0 |
Fair Value, Inputs, Level 1 [Member] | 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 |
Fair Value, Inputs, Level 1 [Member] | Equity Securities | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Equity securities | 11.3 | 8.5 |
Fair Value, Inputs, Level 1 [Member] | Exchange Traded Funds [Member] | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Equity securities | 9.7 | 7.7 |
Fair Value, Inputs, Level 1 [Member] | Partnership Interest [Member] | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Equity securities | 0 | 0 |
Fair Value, Inputs, Level 2 [Member] | | |
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 | 0.8 | |
Investments, Fair Value Disclosure | 6,370.6 | 6,631.4 |
Fair Value, Inputs, Level 2 [Member] | Investments in Fixed Maturities | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Fixed maturities at fair value | 6,347.2 | 6,593.6 |
Fair Value, Inputs, Level 2 [Member] | 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 | 391.3 | 412.7 |
Fair Value, Inputs, Level 2 [Member] | US States and Political Subdivisions Debt Securities [Member] | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Fixed maturities at fair value | 1,339.6 | 1,401.8 |
Fair Value, Inputs, Level 2 [Member] | Foreign Governments | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Fixed maturities at fair value | 4.6 | 3.8 |
Fair Value, Inputs, Level 2 [Member] | Corporate Debt Securities [Member] | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Fixed maturities at fair value | 3,359.3 | 3,513.7 |
Fair Value, Inputs, Level 2 [Member] | Redeemable Preferred Stocks | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Fixed maturities at fair value | 5.8 | 1.2 |
Fair Value, Inputs, Level 2 [Member] | Collateralized Loan Obligations | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Fixed maturities at fair value | 879.1 | 949.8 |
Fair Value, Inputs, Level 2 [Member] | Other Mortgage- and Asset-backed | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Fixed maturities at fair value | 367.5 | 310.6 |
Fair Value, Inputs, Level 2 [Member] | Equity Securities | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Equity securities | 22.6 | 23.1 |
Fair Value, Inputs, Level 2 [Member] | Exchange Traded Funds [Member] | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Equity securities | 0 | 0 |
Fair Value, Inputs, Level 2 [Member] | Partnership Interest [Member] | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Equity securities | 0 | 0 |
Fair Value, Inputs, Level 3 [Member] | | |
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 | 246.7 | 192.3 |
Fair Value, Inputs, Level 3 [Member] | Investments in Fixed Maturities | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Fixed maturities at fair value | 243.3 | 189.5 |
Fair Value, Inputs, Level 3 [Member] | 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 |
Fair Value, Inputs, Level 3 [Member] | US States and Political Subdivisions Debt Securities [Member] | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Fixed maturities at fair value | 0.1 | 0.1 |
Fair Value, Inputs, Level 3 [Member] | Foreign Governments | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Fixed maturities at fair value | 0 | 0 |
Fair Value, Inputs, Level 3 [Member] | Corporate Debt Securities [Member] | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Fixed maturities at fair value | 227.3 | 177.1 |
Fair Value, Inputs, Level 3 [Member] | Redeemable Preferred Stocks | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Fixed maturities at fair value | 4.1 | 7.1 |
Fair Value, Inputs, Level 3 [Member] | Collateralized Loan Obligations | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Fixed maturities at fair value | 6.8 | 0 |
Fair Value, Inputs, Level 3 [Member] | Other Mortgage- and Asset-backed | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Fixed maturities at fair value | 5 | 5.2 |
Fair Value, Inputs, Level 3 [Member] | Equity Securities | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Equity securities | 3.4 | 2.8 |
Fair Value, Inputs, Level 3 [Member] | Exchange Traded Funds [Member] | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Equity securities | 0 | 0 |
Fair Value, Inputs, Level 3 [Member] | Partnership Interest [Member] | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Equity securities | 0 | 0 |
Fair Value Measured at Net Asset Value Per Share | | |
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 [Member] | Preferred Stocks | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Equity securities | 15.9 | 15.6 |
Finance, Insurance and Real Estate [Member] | Common Stock [Member] | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Equity securities | 0.6 | 0.6 |
Finance, Insurance and Real Estate [Member] | Fair Value, Inputs, Level 1 [Member] | Preferred Stocks | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Equity securities | 0 | 0 |
Finance, Insurance and Real Estate [Member] | Fair Value, Inputs, Level 1 [Member] | Common Stock [Member] | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Equity securities | 0.6 | 0.6 |
Finance, Insurance and Real Estate [Member] | Fair Value, Inputs, Level 2 [Member] | Preferred Stocks | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Equity securities | 15.9 | 15.6 |
Finance, Insurance and Real Estate [Member] | Fair Value, Inputs, Level 2 [Member] | Common Stock [Member] | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Equity securities | 0 | 0 |
Finance, Insurance and Real Estate [Member] | Fair Value, Inputs, Level 3 [Member] | Preferred Stocks | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Equity securities | 0 | 0 |
Finance, Insurance and Real Estate [Member] | Fair Value, Inputs, Level 3 [Member] | Common Stock [Member] | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Equity securities | 0 | 0 |
All Industries Other than Finance, Insurance and Real Estate [Member] | Preferred Stocks | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Equity securities | 9 | 9.9 |
All Industries Other than Finance, Insurance and Real Estate [Member] | Common Stock [Member] | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Equity securities | 2.1 | 0.6 |
All Industries Other than Finance, Insurance and Real Estate [Member] | Fair Value, Inputs, Level 1 [Member] | Preferred Stocks | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Equity securities | 0 | 0 |
All Industries Other than Finance, Insurance and Real Estate [Member] | Fair Value, Inputs, Level 1 [Member] | Common Stock [Member] | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Equity securities | 1 | 0.2 |
All Industries Other than Finance, Insurance and Real Estate [Member] | Fair Value, Inputs, Level 2 [Member] | Preferred Stocks | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Equity securities | 6.7 | 7.5 |
All Industries Other than Finance, Insurance and Real Estate [Member] | Fair Value, Inputs, Level 2 [Member] | Common Stock [Member] | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Equity securities | 0 | 0 |
All Industries Other than Finance, Insurance and Real Estate [Member] | Fair Value, Inputs, Level 3 [Member] | Preferred Stocks | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Equity securities | 2.3 | 2.4 |
All Industries Other than Finance, Insurance and Real Estate [Member] | Fair Value, Inputs, Level 3 [Member] | Common Stock [Member] | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Equity securities | 1.1 | 0.4 |
Fair Value, Inputs, Net Asset Value [Member] | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Derivative Asset | | 0 |
Investments, Fair Value Disclosure | 189.3 | 191.4 |
Fair Value, Inputs, Net Asset Value [Member] | Investments in Fixed Maturities | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Fixed maturities at fair value | 0 | 0 |
Fair Value, Inputs, Net Asset Value [Member] | 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 |
Fair Value, Inputs, Net Asset Value [Member] | US States and Political Subdivisions Debt Securities [Member] | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Fixed maturities at fair value | 0 | 0 |
Fair Value, Inputs, Net Asset Value [Member] | Foreign Governments | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Fixed maturities at fair value | 0 | 0 |
Fair Value, Inputs, Net Asset Value [Member] | Corporate Debt Securities [Member] | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Fixed maturities at fair value | 0 | 0 |
Fair Value, Inputs, Net Asset Value [Member] | Redeemable Preferred Stocks | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Fixed maturities at fair value | 0 | 0 |
Fair Value, Inputs, Net Asset Value [Member] | Collateralized Loan Obligations | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Fixed maturities at fair value | 0 | 0 |
Fair Value, Inputs, Net Asset Value [Member] | 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 |
Fair Value, Inputs, Net Asset Value [Member] | Equity Securities | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Equity securities | 189.3 | 191.4 |
Fair Value, Inputs, Net Asset Value [Member] | Exchange Traded Funds [Member] | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Equity securities | 0 | 0 |
Fair Value, Inputs, Net Asset Value [Member] | Partnership Interest [Member] | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Equity securities | 189.3 | 191.4 |
Fair Value, Inputs, Net Asset Value [Member] | Finance, Insurance and Real Estate [Member] | Preferred Stocks | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Equity securities | 0 | 0 |
Fair Value, Inputs, Net Asset Value [Member] | Finance, Insurance and Real Estate [Member] | Common Stock [Member] | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Equity securities | 0 | 0 |
Fair Value, Inputs, Net Asset Value [Member] | All Industries Other than Finance, Insurance and Real Estate [Member] | Preferred Stocks | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Equity securities | 0 | 0 |
Fair Value, Inputs, Net Asset Value [Member] | All Industries Other than Finance, Insurance and Real Estate [Member] | Common Stock [Member] | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Equity securities | $ 0 | $ 0 |