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) (USD $) In Millions, unless otherwise specified	Mar. 31, 2015 security	Dec. 31, 2014 security
Schedule of Held-to-maturity Securities [Line Items]
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
Number of held to maturity securities in unrealized loss positions	71	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	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	10,415
Securities held to maturity 12 months or longer Unrealized Losses	18	173
Total Securities held to maturity with unrealized losses Total Fair Value	8,297	10,679
Total Securities held to maturity with unrealized losses Total Unrealized Losses	48	174
Non-agency commercial mortgage-backed securities [Member]
Schedule of Held-to-maturity Securities [Line Items]
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		660
Total Securities held to maturity with unrealized losses Total Unrealized Losses		$ 14