Investments in Securities Available-for-sale Securities 3 (Details) - USD ($) $ in Millions	Jun. 30, 2016	Dec. 31, 2015
Schedule of Available-for-sale Securities [Line Items]
Less than 12 Consecutive Months, Gross Unrealized Losses	$ (6)	$ (50)
Less than 12 Consecutive Months, Fair Value	278	1,552
12 Consecutive Months or Longer, Gross Unrealized Losses	(30)	(31)
12 Consecutive Months or Longer, Fair Value	652	658
Fannie Mae [Member]
Schedule of Available-for-sale Securities [Line Items]
Less than 12 Consecutive Months, Gross Unrealized Losses	(1)	(8)
Less than 12 Consecutive Months, Fair Value	86	659
12 Consecutive Months or Longer, Gross Unrealized Losses	(20)	(22)
12 Consecutive Months or Longer, Fair Value	514	491
Alt-A private-label securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Less than 12 Consecutive Months, Gross Unrealized Losses	(2)	(1)
Less than 12 Consecutive Months, Fair Value	106	26
12 Consecutive Months or Longer, Gross Unrealized Losses	(4)	(3)
12 Consecutive Months or Longer, Fair Value	49	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	(5)	(4)
12 Consecutive Months or Longer, Fair Value	82	91
CMBS [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	70
12 Consecutive Months or Longer, Gross Unrealized Losses	0
12 Consecutive Months or Longer, Fair Value	0
Mortgage revenue bonds [Member]
Schedule of Available-for-sale Securities [Line Items]
Less than 12 Consecutive Months, Gross Unrealized Losses	(2)	(35)
Less than 12 Consecutive Months, Fair Value	16	631
12 Consecutive Months or Longer, Gross Unrealized Losses	(1)	(2)
12 Consecutive Months or Longer, Fair Value	$ 7	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