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	6,872.0	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	11.0	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		0.0
Cash Flow Hedge Derivative Instrument Assets at Fair Value	0.0
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	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	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	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	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	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	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	11.0	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	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	1,372.0	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	6.0	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	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	0.0
Significant Unobservable Inputs (Level 3)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative Asset		0.0
Cash Flow Hedge Derivative Instrument Assets at Fair Value	0.0
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	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	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	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	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	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	0.0
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	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	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	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	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	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	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		0.0
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	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	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	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	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	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	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	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	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	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	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	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	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	$ 0.0