Held-to-Maturity Securities (Details) - USD ($) $ in Millions	Mar. 31, 2016	Dec. 31, 2015
Schedule of Held-to-maturity Securities [Line Items]
Held-to-maturity securities, amortized cost	$ 14,968	$ 13,013
Held-to-maturity securities, gross unrecognized gains	422	183
Held-to-maturity securities, gross unrecognized losses	(19)	(73)
Held-to-maturity securities, fair value	15,371	13,123
Agency Mortgage-Backed Securities and CMOs [Member]
Schedule of Held-to-maturity Securities [Line Items]
Held-to-maturity securities, amortized cost	12,016	10,353
Held-to-maturity securities, gross unrecognized gains	330	149
Held-to-maturity securities, gross unrecognized losses	(19)	(58)
Held-to-maturity securities, fair value	12,327	10,444
Agency Debentures [Member]
Schedule of Held-to-maturity Securities [Line Items]
Held-to-maturity securities, amortized cost	127	127
Held-to-maturity securities, gross unrecognized gains	1	0
Held-to-maturity securities, gross unrecognized losses	0	(2)
Held-to-maturity securities, fair value	128	125
Agency Debt Securities [Member]
Schedule of Held-to-maturity Securities [Line Items]
Held-to-maturity securities, amortized cost	2,815	2,523
Held-to-maturity securities, gross unrecognized gains	91	34
Held-to-maturity securities, gross unrecognized losses	0	(13)
Held-to-maturity securities, fair value	2,906	2,544
Other Non-Agency Debt Securities [Member]
Schedule of Held-to-maturity Securities [Line Items]
Held-to-maturity securities, amortized cost	10	10
Held-to-maturity securities, fair value	$ 10	$ 10