Held-to-Maturity Securities (Details) - USD ($) $ in Millions	Dec. 31, 2015	Dec. 31, 2014
Schedule of Held-to-maturity Securities [Line Items]
Held-to-maturity securities, amortized cost	$ 13,013	$ 12,248
Held-to-maturity securities, gross unrecognized gains	183	273
Held-to-maturity securities, gross unrecognized losses	(73)	(45)
Held-to-maturity securities, fair value	13,123	12,476
Agency Mortgage-Backed Securities and CMOs [Member]
Schedule of Held-to-maturity Securities [Line Items]
Held-to-maturity securities, amortized cost	10,353	9,793
Held-to-maturity securities, gross unrecognized gains	149	217
Held-to-maturity securities, gross unrecognized losses	(58)	(39)
Held-to-maturity securities, fair value	10,444	9,971
Agency Debentures [Member]
Schedule of Held-to-maturity Securities [Line Items]
Held-to-maturity securities, amortized cost	127	164
Held-to-maturity securities, gross unrecognized gains	0	2
Held-to-maturity securities, gross unrecognized losses	(2)	0
Held-to-maturity securities, fair value	125	166
Agency Debt Securities [Member]
Schedule of Held-to-maturity Securities [Line Items]
Held-to-maturity securities, amortized cost	2,523	2,281
Held-to-maturity securities, gross unrecognized gains	34	54
Held-to-maturity securities, gross unrecognized losses	(13)	(6)
Held-to-maturity securities, fair value	2,544	2,329
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