Property and Casualty Insurance Reserves - Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability (Details) $ in Millions	Dec. 31, 2023 USD ($)
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]
Property and Casualty Insurance Reserves, Net of Reinsurance	$ 2,450.8
Reinsurance Recoverables on Unpaid Losses and Allocated LAE	27.8
Unallocated LAE	201.9
Property and Casualty Insurance Reserves, Gross of Reinsurance	2,680.5
Specialty Personal Automobile Insurance—Liability
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]
Property and Casualty Insurance Reserves, Net of Reinsurance	1,544.9
Reinsurance Recoverables on Unpaid Losses and Allocated LAE	6.0
Specialty Personal Automobile Insurance—Physical Damage
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]
Property and Casualty Insurance Reserves, Net of Reinsurance	11.1
Commercial Automobile Insurance—Liability
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]
Property and Casualty Insurance Reserves, Net of Reinsurance	559.5
Commercial Automobile Insurance—Physical Damage
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]
Property and Casualty Insurance Reserves, Net of Reinsurance	9.7
Preferred Personal Automobile Insurance—Liability
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]
Property and Casualty Insurance Reserves, Net of Reinsurance	226.1
Reinsurance Recoverables on Unpaid Losses and Allocated LAE	18.7
Preferred Personal Automobile Insurance—Physical Damage
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]
Property and Casualty Insurance Reserves, Net of Reinsurance	0.6
Homeowners Insurance
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]
Property and Casualty Insurance Reserves, Net of Reinsurance	61.5
Reinsurance Recoverables on Unpaid Losses and Allocated LAE	0.2
Other
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]
Property and Casualty Insurance Reserves, Net of Reinsurance	37.4
Reinsurance Recoverables on Unpaid Losses and Allocated LAE	$ 2.9