Liability for Unpaid Losses and Loss Adjustment Expense Liability for Unpaid Losses and Loss Adjustment Expenses - Percentage Payout (Details)	Dec. 31, 2016
Homeowners' Insurance Product
Short-duration Insurance Contracts, Historical Claims Duration [Line Items]
Short-duration Insurance Contracts, Historical Claims Duration, Year One	65.40%
Short-duration Insurance Contracts, Historical Claims Duration, Year Two	22.40%
Short-duration Insurance Contracts, Historical Claims Duration, Year Three	4.70%
Short-duration Insurance Contracts, Historical Claims Duration, Year Four	3.60%
Short-duration Insurance Contracts, Historical Claims Duration, Year Five	2.00%
Short-duration Insurance Contracts, Historical Claims Duration, Year Six	1.10%
Short-duration Insurance Contracts, Historical Claims Duration, Year Seven	0.40%
Short-duration Insurance Contracts, Historical Claims Duration, Year Eight	0.20%
Short-duration Insurance Contracts, Historical Claims Duration, Year Nine	0.20%
Short-duration Insurance Contracts, Historical Claims Duration, Year Ten	0.00%
Other Short-duration Insurance Product Line
Short-duration Insurance Contracts, Historical Claims Duration [Line Items]
Short-duration Insurance Contracts, Historical Claims Duration, Year One	48.30%
Short-duration Insurance Contracts, Historical Claims Duration, Year Two	25.70%
Short-duration Insurance Contracts, Historical Claims Duration, Year Three	7.20%
Short-duration Insurance Contracts, Historical Claims Duration, Year Four	6.50%
Short-duration Insurance Contracts, Historical Claims Duration, Year Five	2.80%
Short-duration Insurance Contracts, Historical Claims Duration, Year Six	3.50%
Short-duration Insurance Contracts, Historical Claims Duration, Year Seven	2.50%
Short-duration Insurance Contracts, Historical Claims Duration, Year Eight	1.50%
Short-duration Insurance Contracts, Historical Claims Duration, Year Nine	1.30%
Short-duration Insurance Contracts, Historical Claims Duration, Year Ten	0.70%