Investments in Securities Available-for-sale Securities 3 (Details) - USD ($) $ in Millions	Jun. 30, 2018	Dec. 31, 2017
Schedule of Available-for-sale Securities [Line Items]
Less than 12 Consecutive Months, Gross Unrealized Losses	$ (6)	$ (1)
Less than 12 Consecutive Months, Fair Value	219	134
12 Consecutive Months or Longer, Gross Unrealized Losses	(32)	(30)
12 Consecutive Months or Longer, Fair Value	422	464
Fannie Mae [Member]
Schedule of Available-for-sale Securities [Line Items]
Less than 12 Consecutive Months, Gross Unrealized Losses	(5)	(1)
Less than 12 Consecutive Months, Fair Value	190	134
12 Consecutive Months or Longer, Gross Unrealized Losses	(29)	(26)
12 Consecutive Months or Longer, Fair Value	419	461
Mortgage revenue bonds [Member]
Schedule of Available-for-sale Securities [Line Items]
Less than 12 Consecutive Months, Gross Unrealized Losses	(1)	0
Less than 12 Consecutive Months, Fair Value	29	0
12 Consecutive Months or Longer, Gross Unrealized Losses	(3)	(4)
12 Consecutive Months or Longer, Fair Value	$ 3	$ 3