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Investments in Securities Available-for-sale Securities 3 (Details) - USD ($)
$ in Millions
Dec. 31, 2017
Dec. 31, 2016
Schedule of Available-for-sale Securities [Line Items]    
Less Than 12 Consecutive Months, Gross Unrealized Losses $ (1) $ (9)
Less Than 12 Consecutive Months, Fair Value 134 217
12 Consecutive Months or Longer, Gross Unrealized Losses (30) (31)
12 Consecutive Months or Longer, Fair Value 464 556
Fannie Mae [Member]    
Schedule of Available-for-sale Securities [Line Items]    
Less Than 12 Consecutive Months, Gross Unrealized Losses (1) (2)
Less Than 12 Consecutive Months, Fair Value 134 139
12 Consecutive Months or Longer, Gross Unrealized Losses (26) (26)
12 Consecutive Months or Longer, Fair Value 461 477
Alt-A and subprime private-label securities [Member]    
Schedule of Available-for-sale Securities [Line Items]    
Less Than 12 Consecutive Months, Gross Unrealized Losses   0
Less Than 12 Consecutive Months, Fair Value   0
12 Consecutive Months or Longer, Gross Unrealized Losses   (3)
12 Consecutive Months or Longer, Fair Value   73
Mortgage revenue bonds [Member]    
Schedule of Available-for-sale Securities [Line Items]    
Less Than 12 Consecutive Months, Gross Unrealized Losses 0 (7)
Less Than 12 Consecutive Months, Fair Value 0 78
12 Consecutive Months or Longer, Gross Unrealized Losses (4) (2)
12 Consecutive Months or Longer, Fair Value $ 3 $ 6