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Investments in Securities Available-for-sale Securities 3 (Details) - USD ($)
$ in Millions
Sep. 30, 2018
Dec. 31, 2017
Debt Securities, Available-for-sale [Line Items]    
Less than 12 Consecutive Months, Gross Unrealized Losses $ (5) $ (1)
Less than 12 Consecutive Months, Fair Value 250 134
12 Consecutive Months or Longer, Gross Unrealized Losses (38) (30)
12 Consecutive Months or Longer, Fair Value 475 464
Fannie Mae [Member]    
Debt Securities, Available-for-sale [Line Items]    
Less than 12 Consecutive Months, Gross Unrealized Losses (3) (1)
Less than 12 Consecutive Months, Fair Value 208 134
12 Consecutive Months or Longer, Gross Unrealized Losses (35) (26)
12 Consecutive Months or Longer, Fair Value 472 461
Mortgage revenue bonds [Member]    
Debt Securities, Available-for-sale [Line Items]    
Less than 12 Consecutive Months, Gross Unrealized Losses (2) 0
Less than 12 Consecutive Months, Fair Value 42 0
12 Consecutive Months or Longer, Gross Unrealized Losses (3) (4)
12 Consecutive Months or Longer, Fair Value $ 3 $ 3