Property and Casualty Insurance Reserves - Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability (Details) $ in Millions	Dec. 31, 2021 USD ($)
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]
Property and Casualty Insurance Reserves, Net of Reinsurance	$ 2,491.9
Reinsurance Recoverables on Unpaid Losses and Allocated LAE	41.9
Unallocated LAE	238.9
Property and Casualty Insurance Reserves, Gross of Reinsurance	2,772.7
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	1,722.3
Reinsurance Recoverables on Unpaid Losses and Allocated LAE	10.3
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	63.6
Commercial Automobile Insurance—Liability
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]
Property and Casualty Insurance Reserves, Net of Reinsurance	300.4
Reinsurance Recoverables on Unpaid Losses and Allocated LAE	3.2
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	9.9
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	262.2
Reinsurance Recoverables on Unpaid Losses and Allocated LAE	19.2
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	5.4
Homeowners Insurance
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]
Property and Casualty Insurance Reserves, Net of Reinsurance	81.4
Reinsurance Recoverables on Unpaid Losses and Allocated LAE	5.0
Other
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]
Property and Casualty Insurance Reserves, Net of Reinsurance	46.7
Reinsurance Recoverables on Unpaid Losses and Allocated LAE	$ 4.2