Property and Casualty Insurance Reserves Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability (Details) $ in Millions	Dec. 31, 2017 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	$ 883.3
Reinsurance Recoverable for Unpaid Claims and Claims Adjustments	53.1
Liability for Claims and Claims Adjustment Expense	1,016.8
Short-duration Insurance Contracts, Liability for Unpaid Claims and Claims Adjustment Expense, Accumulated Unallocated Claim Adjustment Expense	80.4
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	229.9
Reinsurance Recoverable for Unpaid Claims and Claims Adjustments	32.5
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	4.1
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	392.3
Reinsurance Recoverable for Unpaid Claims and Claims Adjustments	0.4
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	16.0
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	120.1
Reinsurance Recoverable for Unpaid Claims and Claims Adjustments	11.3
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	120.9
Reinsurance Recoverable for Unpaid Claims and Claims Adjustments	8.8
Other Long-duration Insurance Product Line [Member]
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]
Liability for Claims and Claims Adjustment Expense	$ 0.0