Securities Available for Sale and Securities Held to Maturity (Securities with Unrealized Losses, Aggregated by Category and Period of Continuous Unrealized Loss) (Details) (USD $) In Millions, unless otherwise specified	Mar. 31, 2015		Dec. 31, 2014
Securities Available for Sale and Securities Held to Maturity [Abstract]
Securities available for sale Less than 12 months Fair Value	$ 11,393		$ 11,028
Securities available for sale Less than 12 months Unrealized Losses	25		25
Securities available for sale 12 months or longer Fair Value	4,794		6,807
Securities available for sale 12 months or longer Unrealized Losses	21		58
Total Securities available for sale with unrealized losses Total Fair Value	16,187		17,835
Total Securities available for sale with unrealized losses Total Unrealized Losses	46		83
Securities held to maturity Less than 12 months Fair Value	6,253		264
Securities held to maturity Less than 12 months Unrealized Losses	30		1
Securities held to maturity 12 months or longer Fair Value	2,044		11,075
Securities held to maturity 12 months or longer Unrealized Losses	18		187
Total Securities held to maturity with unrealized losses Total Fair Value	8,297		11,339
Total Securities held to maturity with unrealized losses Total Unrealized Losses	48		188
Securities Less than 12 months Fair Value	17,646	[1]	11,292	[2]
Securities Less than 12 months Unrealized Losses	55	[1]	26	[2]
Securities 12 months or longer Fair Value	6,838	[1]	17,882	[2]
Securities 12 months or longer Unrealized Losses	39	[1]	245	[2]
Total securities with unrealized losses Total Fair Value	24,484	[1]	29,174	[2]
Total securities with unrealized losses Total Unrealized Losses	$ 94	[1]	$ 271	[2]

[1] The number of investment positions with unrealized losses totaled 174 for securities available for sale and 71 for securities held to maturity.

[2] The number of investment positions with unrealized losses totaled 173 for securities available for sale and 111 for securities held to maturity.