Securities (Amortized Cost, Gross Unrealized Gains, Gross Unrealized Losses and Approximate Fair Values of HTM Securities) (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014
HTM securities at amortized cost	$ 18,530	$ 20,240
HTM securities, gross unrealized gains	110	193
HTM securities, gross unrealized losses	121	120
HTM securities, fair value	18,519	20,313
U.S. Treasury [Member]
HTM securities at amortized cost	1,097	1,096
HTM securities, gross unrealized gains	22	23
HTM securities, gross unrealized losses	0	0
HTM securities, fair value	1,119	1,119
GSE [Member]
HTM securities at amortized cost	5,045	5,394
HTM securities, gross unrealized gains	16	17
HTM securities, gross unrealized losses	98	108
HTM securities, fair value	4,963	5,303
Agency MBS [Member]
HTM securities at amortized cost	12,267	13,120
HTM securities, gross unrealized gains	70	137
HTM securities, gross unrealized losses	22	12
HTM securities, fair value	12,315	13,245
States and Political Subdivisions [Member]
HTM securities at amortized cost	63	22
HTM securities, gross unrealized gains	0	2
HTM securities, gross unrealized losses	0	0
HTM securities, fair value	63	24
Other [Member]
HTM securities at amortized cost	58	608
HTM securities, gross unrealized gains	2	14
HTM securities, gross unrealized losses	1	0
HTM securities, fair value	$ 59	$ 622