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Investments in Securities Available-for-sale Securities 3 (Details) - USD ($)
$ in Millions
Dec. 31, 2018
Dec. 31, 2017
Debt Securities, Available-for-sale [Line Items]    
Less Than 12 Consecutive Months, Gross Unrealized Losses $ (1) $ (1)
Less Than 12 Consecutive Months, Fair Value 24 134
12 Consecutive Months or Longer, Gross Unrealized Losses (29) (30)
12 Consecutive Months or Longer, Fair Value 506 464
Fannie Mae [Member]    
Debt Securities, Available-for-sale [Line Items]    
Less Than 12 Consecutive Months, Gross Unrealized Losses 0 (1)
Less Than 12 Consecutive Months, Fair Value 0 134
12 Consecutive Months or Longer, Gross Unrealized Losses (26) (26)
12 Consecutive Months or Longer, Fair Value 487 461
Mortgage revenue bonds [Member]    
Debt Securities, Available-for-sale [Line Items]    
Less Than 12 Consecutive Months, Gross Unrealized Losses (1) 0
Less Than 12 Consecutive Months, Fair Value 24 0
12 Consecutive Months or Longer, Gross Unrealized Losses (3) (4)
12 Consecutive Months or Longer, Fair Value $ 19 $ 3