Property and Casualty Insurance Reserves - Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability (Details) $ in Millions	Dec. 31, 2020 USD ($)
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]
Property and Casualty Insurance Reserves, Net of Reinsurance	$ 1,756.7
Reinsurance Recoverables on Unpaid Losses and Allocated LAE	50.1
Unallocated LAE	175.7
Property and Casualty Insurance Reserves, Gross of Reinsurance	1,982.5
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	1,112.5
Reinsurance Recoverables on Unpaid Losses and Allocated LAE	9.3
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	53.5
Commercial Automobile Insurance—Liability
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]
Property and Casualty Insurance Reserves, Net of Reinsurance	209.1
Reinsurance Recoverables on Unpaid Losses and Allocated LAE	5.8
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	6.3
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	232.6
Reinsurance Recoverables on Unpaid Losses and Allocated LAE	23.2
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	4.4
Homeowners Insurance
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]
Property and Casualty Insurance Reserves, Net of Reinsurance	88.4
Reinsurance Recoverables on Unpaid Losses and Allocated LAE	7.6
Other
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]
Property and Casualty Insurance Reserves, Net of Reinsurance	49.9
Reinsurance Recoverables on Unpaid Losses and Allocated LAE	$ 4.2