Property and Casualty Insurance Reserves Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability (Details) $ in Millions	Dec. 31, 2016 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	$ 807.5
Reinsurance Recoverable for Unpaid Claims and Claims Adjustments	47.0
Liability for Claims and Claims Adjustment Expense	931.4
Short-duration Insurance Contracts, Liability for Unpaid Claims and Claims Adjustment Expense, Accumulated Unallocated Claim Adjustment Expense	76.9
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	219.3
Reinsurance Recoverable for Unpaid Claims and Claims Adjustments	28.3
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	0.8
Reinsurance Recoverable for Unpaid Claims and Claims Adjustments	0.0
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	371.7
Reinsurance Recoverable for Unpaid Claims and Claims Adjustments	10.6
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	9.3
Reinsurance Recoverable for Unpaid Claims and Claims Adjustments	(0.6)
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	81.7
Reinsurance Recoverable for Unpaid Claims and Claims Adjustments	0.0
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	124.7
Reinsurance Recoverable for Unpaid Claims and Claims Adjustments	8.7
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