Property and Casualty Insurance Reserves - Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability (Details) $ in Millions	Dec. 31, 2019 USD ($)
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]
Property and Casualty Insurance Reserves, Net of Reinsurance	$ 1,725.1
Reinsurance Recoverables on Unpaid Losses and Allocated LAE	65.6
Unallocated LAE	179.1
Property and Casualty Insurance Reserves, Gross of Reinsurance	1,969.8
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	1,140.9
Reinsurance Recoverables on Unpaid Losses and Allocated LAE	8.7
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	42.2
Commercial Automobile Insurance—Liability
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]
Property and Casualty Insurance Reserves, Net of Reinsurance	191.2
Reinsurance Recoverables on Unpaid Losses and Allocated LAE	9.4
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	3.0
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	212.9
Reinsurance Recoverables on Unpaid Losses and Allocated LAE	25.2
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	5.1
Homeowners Insurance
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]
Property and Casualty Insurance Reserves, Net of Reinsurance	72.3
Reinsurance Recoverables on Unpaid Losses and Allocated LAE	15.5
Other
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]
Property and Casualty Insurance Reserves, Net of Reinsurance	57.5
Reinsurance Recoverables on Unpaid Losses and Allocated LAE	$ 6.8