Securities Available for Sale and Securities Held to Maturity (Held To Maturity Securities with Unrealized Losses, Aggregated by Category and Period of Continuous Unrealized Loss) (Details) $ in Millions	Dec. 31, 2015 USD ($) security	Dec. 31, 2014 USD ($) security
Schedule of Held-to-maturity Securities [Line Items]
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
Number of held to maturity securities in unrealized loss positions | security	286	111
U.S. agency mortgage-backed securities [Member]
Schedule of Held-to-maturity Securities [Line Items]
Securities held to maturity Less than 12 months Fair Value	$ 24,219	$ 264
Securities held to maturity Less than 12 months Unrealized Losses	253	1
Securities held to maturity 12 months or longer Fair Value	1,842	10,415
Securities held to maturity 12 months or longer Unrealized Losses	40	173
Total Securities held to maturity with unrealized losses Total Fair Value	26,061	10,679
Total Securities held to maturity with unrealized losses Total Unrealized Losses	293	174
Non-agency commercial mortgage-backed securities [Member]
Schedule of Held-to-maturity Securities [Line Items]
Securities held to maturity Less than 12 months Fair Value	729
Securities held to maturity Less than 12 months Unrealized Losses	20
Securities held to maturity 12 months or longer Fair Value		660
Securities held to maturity 12 months or longer Unrealized Losses		14
Total Securities held to maturity with unrealized losses Total Fair Value	729	660
Total Securities held to maturity with unrealized losses Total Unrealized Losses	20	$ 14
U.S. Treasury securities [Member]
Schedule of Held-to-maturity Securities [Line Items]
Securities held to maturity Less than 12 months Fair Value	220
Securities held to maturity Less than 12 months Unrealized Losses	3
Total Securities held to maturity with unrealized losses Total Fair Value	220
Total Securities held to maturity with unrealized losses Total Unrealized Losses	$ 3