Securities (Amortized Cost, Gross Unrealized Gains, Gross Unrealized Losses and Approximate Fair Values of Securities Held to Maturity) (Details) (USD $) In Millions, unless otherwise specified	Sep. 30, 2013	Dec. 31, 2012
HTM securities, amortized cost	$ 13,529	$ 13,594
HTM securities, gross unrealized gains	42	260
HTM securities, gross unrealized losses	390	6
HTM securities, fair value	13,181	13,848
GSE Securities [Member]
HTM securities, amortized cost	5,382	3,808
HTM securities, gross unrealized gains	3	17
HTM securities, gross unrealized losses	328	1
HTM securities, fair value	5,057	3,824
MBS Issued by GSE [Member]
HTM securities, amortized cost	7,668	9,273
HTM securities, gross unrealized gains	27	238
HTM securities, gross unrealized losses	61	1
HTM securities, fair value	7,634	9,510
States and Political Subdivisions [Member]
HTM securities, amortized cost	32	34
HTM securities, gross unrealized gains	1	1
HTM securities, gross unrealized losses	1	1
HTM securities, fair value	32	34
Other Securities [Member]
HTM securities, amortized cost	447	479
HTM securities, gross unrealized gains	11	4
HTM securities, gross unrealized losses	0	3
HTM securities, fair value	$ 458	$ 480