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	Dec. 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	$ 38,165	$ 11,028
Securities available for sale Less than 12 months Unrealized Losses	341	25
Securities available for sale 12 months or longer Fair Value	6,644	6,807
Securities available for sale 12 months or longer Unrealized Losses	91	58
Total Securities available for sale with unrealized losses Total Fair Value	44,809	17,835
Total Securities available for sale with unrealized losses Total Unrealized Losses	432	83
Securities held to maturity Less than 12 months Fair Value	25,168	264
Securities held to maturity Less than 12 months Unrealized Losses	276	1
Securities held to maturity 12 months or longer Fair Value	1,842	11,075
Securities held to maturity 12 months or longer Unrealized Losses	40	187
Total Securities held to maturity with unrealized losses Total Fair Value	27,010	11,339
Total Securities held to maturity with unrealized losses Total Unrealized Losses	316	188
Securities Less than 12 months Fair Value	63,333	11,292
Securities Less than 12 months Unrealized Losses	617	26
Securities 12 months or longer Fair Value	8,486	17,882
Securities 12 months or longer Unrealized Losses	131	245
Total securities with unrealized losses Total Fair Value	71,819	29,174
Total securities with unrealized losses Total Unrealized Losses	$ 748	$ 271