Held-to-Maturity Securities (Details) - USD ($) $ in Millions	Jun. 30, 2017	Dec. 31, 2016
Schedule of Held-to-maturity Securities [Line Items]
Held-to-maturity securities, amortized cost	$ 21,502	$ 15,751
Held-to-maturity securities, gross unrecognized gains	199	149
Held-to-maturity securities, gross unrecognized losses	(136)	(184)
Held-to-maturity securities, fair value	21,565	15,716
Agency Mortgage-Backed Securities [Member]
Schedule of Held-to-maturity Securities [Line Items]
Held-to-maturity securities, amortized cost	18,426	12,868
Held-to-maturity securities, gross unrecognized gains	168	123
Held-to-maturity securities, gross unrecognized losses	(115)	(152)
Held-to-maturity securities, fair value	18,479	12,839
Agency Debentures [Member]
Schedule of Held-to-maturity Securities [Line Items]
Held-to-maturity securities, amortized cost	291	29
Held-to-maturity securities, gross unrecognized gains	1	0
Held-to-maturity securities, gross unrecognized losses	(1)	0
Held-to-maturity securities, fair value	291	29
Agency Debt Securities [Member]
Schedule of Held-to-maturity Securities [Line Items]
Held-to-maturity securities, amortized cost	2,778	2,854
Held-to-maturity securities, gross unrecognized gains	30	26
Held-to-maturity securities, gross unrecognized losses	(20)	(32)
Held-to-maturity securities, fair value	2,788	$ 2,848
Other Non-Agency Debt Securities [Member]
Schedule of Held-to-maturity Securities [Line Items]
Held-to-maturity securities, amortized cost	7
Held-to-maturity securities, fair value	$ 7