Property and Casualty Insurance Reserves - Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability (Details) $ in Millions	Dec. 31, 2022 USD ($)
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]
Property and Casualty Insurance Reserves, Net of Reinsurance	$ 2,507.8
Reinsurance Recoverables on Unpaid Losses and Allocated LAE	39.6
Unallocated LAE	209.5
Property and Casualty Insurance Reserves, Gross of Reinsurance	2,756.9
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	1,642.5
Reinsurance Recoverables on Unpaid Losses and Allocated LAE	1.5
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	62.7
Commercial Automobile Insurance—Liability
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]
Property and Casualty Insurance Reserves, Net of Reinsurance	414.3
Reinsurance Recoverables on Unpaid Losses and Allocated LAE	19.2
Specialty Personal Automobile Insurance—Physical Damage
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]
Reinsurance Recoverables on Unpaid Losses and Allocated LAE	0.0
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	8.3
Reinsurance Recoverables on Unpaid Losses and Allocated LAE	0.0
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	254.8
Reinsurance Recoverables on Unpaid Losses and Allocated LAE	10.1
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	2.8
Reinsurance Recoverables on Unpaid Losses and Allocated LAE	0.0
Homeowners Insurance
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]
Property and Casualty Insurance Reserves, Net of Reinsurance	78.3
Reinsurance Recoverables on Unpaid Losses and Allocated LAE	4.9
Other
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]
Property and Casualty Insurance Reserves, Net of Reinsurance	44.1
Reinsurance Recoverables on Unpaid Losses and Allocated LAE	$ 3.9