Investments in Securities Available-for-sale Securities 3 (Details) - USD ($) $ in Millions	Sep. 30, 2019	Dec. 31, 2018
Debt Securities, Available-for-sale [Line Items]
Less than 12 Consecutive Months, Gross Unrealized Losses	$ 2	$ 1
Less than 12 Consecutive Months, Fair Value	273	24
12 Consecutive Months or Longer, Gross Unrealized Losses	(11)	(29)
12 Consecutive Months or Longer, Fair Value	353	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	(8)	(26)
12 Consecutive Months or Longer, Fair Value	348	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	5	19
Other mortgage-related securities [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	273	0
12 Consecutive Months or Longer, Gross Unrealized Losses	0	0
12 Consecutive Months or Longer, Fair Value	$ 0	$ 0