Securities (Amortized Cost, Gross Unrealized Gains, Gross Unrealized Losses and Approximate Fair Values of HTM Securities) (Details) (USD $) In Millions, unless otherwise specified	12 Months Ended
	Dec. 31, 2014	Dec. 31, 2013
HTM securities	$ 20,240	$ 18,101
HTM securities, gross unrealized gains	193	54
HTM securities, gross unrealized losses	120	625
HTM securities, fair value	20,313	17,530
U.S. Treasury [Member]
HTM securities	1,096	392
HTM securities, gross unrealized gains	23	0
HTM securities, gross unrealized losses	0	8
HTM securities, fair value	1,119	384
GSE [Member]
HTM securities	5,394	5,603
HTM securities, gross unrealized gains	17	2
HTM securities, gross unrealized losses	108	397
HTM securities, fair value	5,303	5,208
MBS Issued by GSE [Member]
HTM securities	13,120	11,636
HTM securities, gross unrealized gains	137	38
HTM securities, gross unrealized losses	12	220
HTM securities, fair value	13,245	11,454
States and Political Subdivisions [Member]
HTM securities	22	33
HTM securities, gross unrealized gains	2	2
HTM securities, gross unrealized losses	0	0
HTM securities, fair value	24	35
Other [Member]
HTM securities	608	437
HTM securities, gross unrealized gains	14	12
HTM securities, gross unrealized losses	0	0
HTM securities, fair value	$ 622	$ 449