Property and Casualty Insurance Reserves Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability (Details) $ in Millions	Dec. 31, 2018 USD ($)
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]
Short-duration Insurance Contracts, Liability for Unpaid Claims and Allocated Claim Adjustment Expense, Net	$ 1,601.2
Reinsurance Recoverable for Unpaid Claims and Claims Adjustments	101.9
Liability for Claims and Claims Adjustment Expense	1,874.9
Short-duration Insurance Contracts, Liability for Unpaid Claims and Claims Adjustment Expense, Accumulated Unallocated Claim Adjustment Expense	171.8
Preferred Personal Automobile Liability Insurance [Member]
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]
Short-duration Insurance Contracts, Liability for Unpaid Claims and Allocated Claim Adjustment Expense, Net	218.0
Reinsurance Recoverable for Unpaid Claims and Claims Adjustments	26.9
Preferred Personal Automobile Physical Damage Insurance [Member]
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]
Short-duration Insurance Contracts, Liability for Unpaid Claims and Allocated Claim Adjustment Expense, Net	6.2
Reinsurance Recoverable for Unpaid Claims and Claims Adjustments	0.1
Non-standard Personal Automobile Liability Insurance [Member]
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]
Short-duration Insurance Contracts, Liability for Unpaid Claims and Allocated Claim Adjustment Expense, Net	1,006.7
Reinsurance Recoverable for Unpaid Claims and Claims Adjustments	9.2
Non-standard Personal Automobile Physical Damage Insurance [Member]
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]
Short-duration Insurance Contracts, Liability for Unpaid Claims and Allocated Claim Adjustment Expense, Net	35.5
Reinsurance Recoverable for Unpaid Claims and Claims Adjustments	0.0
Commercial Automobile Insurance-Liability [Member]
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]
Short-duration Insurance Contracts, Liability for Unpaid Claims and Allocated Claim Adjustment Expense, Net	175.6
Reinsurance Recoverable for Unpaid Claims and Claims Adjustments	13.1
Commercial Automobile Insurance - Physical Damage [Member]
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]
Short-duration Insurance Contracts, Liability for Unpaid Claims and Allocated Claim Adjustment Expense, Net	1.9
Reinsurance Recoverable for Unpaid Claims and Claims Adjustments	0.0
Homeowners [Member]
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]
Short-duration Insurance Contracts, Liability for Unpaid Claims and Allocated Claim Adjustment Expense, Net	94.1
Reinsurance Recoverable for Unpaid Claims and Claims Adjustments	42.7
Other Short-duration Insurance Product Line [Member]
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]
Short-duration Insurance Contracts, Liability for Unpaid Claims and Allocated Claim Adjustment Expense, Net	63.2
Reinsurance Recoverable for Unpaid Claims and Claims Adjustments	$ 9.9