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Investments in Securities - Available-for-sale Securities 2 (Details) - Fannie Mae - USD ($)
$ in Millions
Jun. 30, 2021
Dec. 31, 2020
Debt Securities, Available-for-sale [Line Items]    
Less Than 12 Consecutive Months, Gross Unrealized Losses in AOCI $ 0 $ (1)
Less than 12 Consecutive Months, Fair Value 0 40
12 Consecutive Months or Longer, Gross Unrealized Losses (7) (11)
12 Consecutive Months or Longer, Fair Value $ 129 $ 94