Investments in Securities Available-for-sale Securities 3 (Details) - USD ($) $ in Millions	Dec. 31, 2019	Dec. 31, 2018
Debt Securities, Available-for-sale [Line Items]
Less Than 12 Consecutive Months, Gross Unrealized Losses	$ (1)	$ (1)
Less Than 12 Consecutive Months, Fair Value	130	24
12 Consecutive Months or Longer, Gross Unrealized Losses	(13)	(29)
12 Consecutive Months or Longer, Fair Value	340	506
Fannie Mae [Member]
Debt Securities, Available-for-sale [Line Items]
Less Than 12 Consecutive Months, Gross Unrealized Losses	0	0
Less Than 12 Consecutive Months, Fair Value	0	0
12 Consecutive Months or Longer, Gross Unrealized Losses	(10)	(26)
12 Consecutive Months or Longer, Fair Value	337	487
Mortgage revenue bonds [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	24
12 Consecutive Months or Longer, Gross Unrealized Losses	(3)	(3)
12 Consecutive Months or Longer, Fair Value	3	$ 19
Other mortgage-related securities [Member]
Debt Securities, Available-for-sale [Line Items]
Less Than 12 Consecutive Months, Gross Unrealized Losses	(1)
Less Than 12 Consecutive Months, Fair Value	130
12 Consecutive Months or Longer, Gross Unrealized Losses	0
12 Consecutive Months or Longer, Fair Value	$ 0