Reserves for Insurance Losses and Loss Adjustment Expenses Reserves for Insurance Losses and Loss Adjustment Expenses (Short-duration Contracts, Schedule of Historical Claims Duration) (Details)	Dec. 31, 2017
Reserves for Insurance Losses and Loss Adjustment Expenses (Short-duration Contracts, Schedule of Historical Claims Duration) [Abstract]
Short-duration Insurance Contracts, Historical Claims Duration, Year One	93.00%
Short-duration Insurance Contracts, Historical Claims Duration, Year Two	5.70%
Short-duration Insurance Contracts, Historical Claims Duration, Year Three	0.40%
Short-duration Insurance Contracts, Historical Claims Duration, Year Four	0.30%
Short-duration Insurance Contracts, Historical Claims Duration, Year Five	0.20%
Short-duration Insurance Contracts, Historical Claims Duration, Year Six	0.40%