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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