Investment Securities (Cash Flow Model Inputs Used To Calculate Credit Losses) (Details)	Sep. 30, 2017
Minimum [Member]
Investment [Line Items]
Prepayment CPR	3.00%
Projected cumulative default	13.00%
Credit support	0.00%
Loss severity	16.00%
Maximum [Member]
Investment [Line Items]
Prepayment CPR	25.00%
Projected cumulative default	50.00%
Credit support	47.00%
Loss severity	63.00%