Investments in Securities Available-for-sale Securities 3 (Details) (USD $) In Millions, unless otherwise specified	Sep. 30, 2014	Dec. 31, 2013
Schedule of Available-for-sale Securities [Line Items]
Less than 12 Consecutive Months, Gross Unrealized Losses	$ (11)	$ (223)
Less than 12 Consecutive Months, Fair Value	531	3,580
12 Consecutive Months or Longer, Gross Unrealized Losses	(244)	(660)
12 Consecutive Months or Longer, Fair Value	2,798	3,802
Fannie Mae [Member]
Schedule of Available-for-sale Securities [Line Items]
Less than 12 Consecutive Months, Gross Unrealized Losses	(5)	(40)
Less than 12 Consecutive Months, Fair Value	335	975
12 Consecutive Months or Longer, Gross Unrealized Losses	(25)	(4)
12 Consecutive Months or Longer, Fair Value	624	126
Alt-A private-label securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Less than 12 Consecutive Months, Gross Unrealized Losses	(3)	(12)
Less than 12 Consecutive Months, Fair Value	90	490
12 Consecutive Months or Longer, Gross Unrealized Losses	(14)	(30)
12 Consecutive Months or Longer, Fair Value	113	308
Subprime private-label securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Less than 12 Consecutive Months, Gross Unrealized Losses	0	(24)
Less than 12 Consecutive Months, Fair Value	0	448
12 Consecutive Months or Longer, Gross Unrealized Losses	(25)	(131)
12 Consecutive Months or Longer, Fair Value	485	1,332
Mortgage revenue bonds [Member]
Schedule of Available-for-sale Securities [Line Items]
Less than 12 Consecutive Months, Gross Unrealized Losses	(3)	(147)
Less than 12 Consecutive Months, Fair Value	106	1,662
12 Consecutive Months or Longer, Gross Unrealized Losses	(22)	(277)
12 Consecutive Months or Longer, Fair Value	543	970
Other mortgage-related 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	5
12 Consecutive Months or Longer, Gross Unrealized Losses	(158)	(218)
12 Consecutive Months or Longer, Fair Value	$ 1,033	$ 1,066