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Available-for-sale Securities (Summary of Available-for-sale MBS issued by Members or Affiliates of Members) (Details) - USD ($)
$ in Millions
Jun. 30, 2019
Dec. 31, 2018
Debt Securities, Available-for-sale [Line Items]    
Available-for-sale Securities $ 777 $ 865
M B S Issued by Members or Affiliates of Members [Member]    
Debt Securities, Available-for-sale [Line Items]    
Amortized cost of available-for-sale securities 498 539
Other-than-temporary Impairment Recognized in Accumulated Other Comprehensive Income [1] 0 0
Gross unrealized gains on available-for-sale securities 41 54
Gross unrealized losses on available-for-sale securities 0 0
Available-for-sale Securities $ 539 $ 593
[1] Amounts represent the non-credit portion of an other-than-temporary impairment during the life of the security.