Investments in Securities Available-for-sale Securities 3 (Details) - USD ($) $ in Millions	Dec. 31, 2016	Dec. 31, 2015
Schedule of Available-for-sale Securities [Line Items]
Less Than 12 Consecutive Months, Gross Unrealized Losses	$ (9)	$ (50)
Less Than 12 Consecutive Months, Fair Value	217	1,552
12 Consecutive Months or Longer, Gross Unrealized Losses	(31)	(31)
12 Consecutive Months or Longer, Fair Value	556	658
Fannie Mae [Member]
Schedule of Available-for-sale Securities [Line Items]
Less Than 12 Consecutive Months, Gross Unrealized Losses	(2)	(8)
Less Than 12 Consecutive Months, Fair Value	139	659
12 Consecutive Months or Longer, Gross Unrealized Losses	(26)	(22)
12 Consecutive Months or Longer, Fair Value	477	491
Alt-A private-label securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Less Than 12 Consecutive Months, Gross Unrealized Losses		(1)
Less Than 12 Consecutive Months, Fair Value		26
12 Consecutive Months or Longer, Gross Unrealized Losses		(3)
12 Consecutive Months or Longer, Fair Value		54
Subprime private-label securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Less Than 12 Consecutive Months, Gross Unrealized Losses	0	0
Less Than 12 Consecutive Months, Fair Value	0	12
12 Consecutive Months or Longer, Gross Unrealized Losses	(3)	(4)
12 Consecutive Months or Longer, Fair Value	73	91
Mortgage revenue bonds [Member]
Schedule of Available-for-sale Securities [Line Items]
Less Than 12 Consecutive Months, Gross Unrealized Losses	(7)	(35)
Less Than 12 Consecutive Months, Fair Value	78	631
12 Consecutive Months or Longer, Gross Unrealized Losses	(2)	(2)
12 Consecutive Months or Longer, Fair Value	$ 6	22
Other mortgage-related securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Less Than 12 Consecutive Months, Gross Unrealized Losses		(6)
Less Than 12 Consecutive Months, Fair Value		224
12 Consecutive Months or Longer, Gross Unrealized Losses		0
12 Consecutive Months or Longer, Fair Value		$ 0