Investment Securities (Cash Flow Model Inputs Used To Calculate Credit Losses) (Details)	Mar. 31, 2018
Minimum [Member]
Investment [Line Items]
Prepayment CPR	0.00%
Projected cumulative default	15.00%
Credit support	0.00%
Loss severity	15.00%
Maximum [Member]
Investment [Line Items]
Prepayment CPR	25.00%
Projected cumulative default	52.00%
Credit support	63.00%
Loss severity	63.00%