Investments in Securities Available-for-sale Securities 3 (Details) - USD ($) $ in Millions	Mar. 31, 2018	Dec. 31, 2017
Schedule of Available-for-sale Securities [Line Items]
Less than 12 Consecutive Months, Gross Unrealized Losses	$ (13)	$ (1)
Less than 12 Consecutive Months, Fair Value	202	134
12 Consecutive Months or Longer, Gross Unrealized Losses	(35)	(30)
12 Consecutive Months or Longer, Fair Value	428	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	157	134
12 Consecutive Months or Longer, Gross Unrealized Losses	(32)	(26)
12 Consecutive Months or Longer, Fair Value	425	461
Mortgage revenue bonds [Member]
Schedule of Available-for-sale Securities [Line Items]
Less than 12 Consecutive Months, Gross Unrealized Losses	(8)	0
Less than 12 Consecutive Months, Fair Value	45	0
12 Consecutive Months or Longer, Gross Unrealized Losses	(3)	(4)
12 Consecutive Months or Longer, Fair Value	$ 3	$ 3