Securities (Gross Unrealized Losses and Fair Values of Investments by Investment Category and Length of Time) (Details) (USD $) In Millions, unless otherwise specified	12 Months Ended
	Dec. 31, 2014	Dec. 31, 2013
Fair value of temporarily impaired AFS securities, less than 12 months	$ 2,298	$ 10,525
Unrealized losses on temporarily impaired AFS securities, less than 12 months	19	415
Fair value of temporarily impaired AFS securities, 12 months or more	7,327	2,376
Unrealized losses on temporarily impaired AFS securities, 12 months or more	337	223
Total fair value of temporarily impaired AFS securities	9,625	12,901
Total unrealized losses on temporarily impaired AFS securities	356	638
Fair value of temporarily impaired HTM securities, less than 12 months	2,225	14,180
Unrealized losses on temporarily impaired HTM securities, less than 12 months	10	624
Fair value of temporarily impaired HTM securities, 12 months or more	4,768	48
Unrealized losses on temporarily impaired HTM securities, 12 months or more	110	1
Total fair value of temporarily impaired HTM securities	6,993	14,228
Total unrealized losses on temporarily impaired HTM securities	120	625
U.S. Treasury [Member]
Fair value of temporarily impaired HTM securities, less than 12 months		384
Unrealized losses on temporarily impaired HTM securities, less than 12 months		8
Fair value of temporarily impaired HTM securities, 12 months or more		0
Unrealized losses on temporarily impaired HTM securities, 12 months or more		0
Total fair value of temporarily impaired HTM securities		384
Total unrealized losses on temporarily impaired HTM securities		8
GSE [Member]
Fair value of temporarily impaired HTM securities, less than 12 months	896	4,996
Unrealized losses on temporarily impaired HTM securities, less than 12 months	5	397
Fair value of temporarily impaired HTM securities, 12 months or more	3,968	0
Unrealized losses on temporarily impaired HTM securities, 12 months or more	103	0
Total fair value of temporarily impaired HTM securities	4,864	4,996
Total unrealized losses on temporarily impaired HTM securities	108	397
MBS Issued by GSE [Member]
Fair value of temporarily impaired AFS securities, less than 12 months	2,285	10,259
Unrealized losses on temporarily impaired AFS securities, less than 12 months	19	406
Fair value of temporarily impaired AFS securities, 12 months or more	6,878	1,935
Unrealized losses on temporarily impaired AFS securities, 12 months or more	278	140
Total fair value of temporarily impaired AFS securities	9,163	12,194
Total unrealized losses on temporarily impaired AFS securities	297	546
Fair value of temporarily impaired HTM securities, less than 12 months	1,329	8,800
Unrealized losses on temporarily impaired HTM securities, less than 12 months	5	219
Fair value of temporarily impaired HTM securities, 12 months or more	800	48
Unrealized losses on temporarily impaired HTM securities, 12 months or more	7	1
Total fair value of temporarily impaired HTM securities	2,129	8,848
Total unrealized losses on temporarily impaired HTM securities	12	220
States and Political Subdivisions [Member]
Fair value of temporarily impaired AFS securities, less than 12 months	13	232
Unrealized losses on temporarily impaired AFS securities, less than 12 months	0	8
Fair value of temporarily impaired AFS securities, 12 months or more	449	441
Unrealized losses on temporarily impaired AFS securities, 12 months or more	59	83
Total fair value of temporarily impaired AFS securities	462	673
Total unrealized losses on temporarily impaired AFS securities	59	91
Securities Acquired from FDIC [Member]
Fair value of temporarily impaired AFS securities, less than 12 months		34
Unrealized losses on temporarily impaired AFS securities, less than 12 months		1
Fair value of temporarily impaired AFS securities, 12 months or more		0
Unrealized losses on temporarily impaired AFS securities, 12 months or more		0
Total fair value of temporarily impaired AFS securities		34
Total unrealized losses on temporarily impaired AFS securities		$ 1