Investments in Securities Available-for-sale Securities 3 (Details) - USD ($) $ in Millions	Sep. 30, 2017	Dec. 31, 2016
Schedule of Available-for-sale Securities [Line Items]
Less than 12 Consecutive Months, Gross Unrealized Losses	$ (2)	$ (9)
Less than 12 Consecutive Months, Fair Value	150	217
12 Consecutive Months or Longer, Gross Unrealized Losses	(30)	(31)
12 Consecutive Months or Longer, Fair Value	445	556
Fannie Mae [Member]
Schedule of Available-for-sale Securities [Line Items]
Less than 12 Consecutive Months, Gross Unrealized Losses	(2)	(2)
Less than 12 Consecutive Months, Fair Value	150	139
12 Consecutive Months or Longer, Gross Unrealized Losses	(24)	(26)
12 Consecutive Months or Longer, Fair Value	430	477
Alt-A and subprime private-label securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Less than 12 Consecutive Months, Gross Unrealized Losses		0
Less than 12 Consecutive Months, Fair Value		0
12 Consecutive Months or Longer, Gross Unrealized Losses		(3)
12 Consecutive Months or Longer, Fair Value		73
Mortgage revenue bonds [Member]
Schedule of Available-for-sale Securities [Line Items]
Less than 12 Consecutive Months, Gross Unrealized Losses	0	(7)
Less than 12 Consecutive Months, Fair Value	0	78
12 Consecutive Months or Longer, Gross Unrealized Losses	(6)	(2)
12 Consecutive Months or Longer, Fair Value	$ 15	$ 6