Held-to-Maturity Securities (Details) - USD ($) $ in Millions	Dec. 31, 2017	Dec. 31, 2016
Schedule of Held-to-maturity Securities [Line Items]
Held-to-maturity securities, amortized cost	$ 23,839	$ 15,751
Held-to-maturity securities, gross unrecognized gains	110	149
Held-to-maturity securities, gross unrecognized losses	(230)	(184)
Held-to-maturity securities, fair value	23,719	15,716
Agency Mortgage-Backed Securities [Member]
Schedule of Held-to-maturity Securities [Line Items]
Held-to-maturity securities, amortized cost	20,502	12,868
Held-to-maturity securities, gross unrecognized gains	95	123
Held-to-maturity securities, gross unrecognized losses	(193)	(152)
Held-to-maturity securities, fair value	20,404	12,839
Agency Debentures [Member]
Schedule of Held-to-maturity Securities [Line Items]
Held-to-maturity securities, amortized cost	710	29
Held-to-maturity securities, gross unrecognized gains	0	0
Held-to-maturity securities, gross unrecognized losses	(2)	0
Held-to-maturity securities, fair value	708	29
Agency Debt Securities [Member]
Schedule of Held-to-maturity Securities [Line Items]
Held-to-maturity securities, amortized cost	2,615	2,854
Held-to-maturity securities, gross unrecognized gains	15	26
Held-to-maturity securities, gross unrecognized losses	(35)	(32)
Held-to-maturity securities, fair value	2,595	$ 2,848
Other Non-Agency Debt Securities [Member]
Schedule of Held-to-maturity Securities [Line Items]
Held-to-maturity securities, amortized cost	12
Held-to-maturity securities, fair value	$ 12