Investments in Securities Available-for-sale Securities 3 (Details) - USD ($) $ in Millions	Dec. 31, 2015	Dec. 31, 2014
Schedule of Available-for-sale Securities [Line Items]
Less Than 12 Consecutive Months, Gross Unrealized Losses	$ (50)	$ (4)
Less Than 12 Consecutive Months, Fair Value	1,552	339
12 Consecutive Months or Longer, Gross Unrealized Losses	(31)	(203)
12 Consecutive Months or Longer, Fair Value	658	2,375
Fannie Mae [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	659	113
12 Consecutive Months or Longer, Gross Unrealized Losses	(22)	(19)
12 Consecutive Months or Longer, Fair Value	491	627
Alt-A private-label securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Less Than 12 Consecutive Months, Gross Unrealized Losses	(1)	(2)
Less Than 12 Consecutive Months, Fair Value	26	171
12 Consecutive Months or Longer, Gross Unrealized Losses	(3)	(13)
12 Consecutive Months or Longer, Fair Value	54	112
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	12	0
12 Consecutive Months or Longer, Gross Unrealized Losses	(4)	(24)
12 Consecutive Months or Longer, Fair Value	91	460
Mortgage revenue bonds [Member]
Schedule of Available-for-sale Securities [Line Items]
Less Than 12 Consecutive Months, Gross Unrealized Losses	(35)	(2)
Less Than 12 Consecutive Months, Fair Value	631	47
12 Consecutive Months or Longer, Gross Unrealized Losses	(2)	(11)
12 Consecutive Months or Longer, Fair Value	22	155
Other mortgage-related securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Less Than 12 Consecutive Months, Gross Unrealized Losses	(6)	0
Less Than 12 Consecutive Months, Fair Value	224	8
12 Consecutive Months or Longer, Gross Unrealized Losses	0	(136)
12 Consecutive Months or Longer, Fair Value	$ 0	$ 1,021