Investments in Securities Available-for-sale Securities 3 (Details) (USD $) In Millions, unless otherwise specified	Sep. 30, 2012	Dec. 31, 2011
Schedule of Available-for-sale Securities [Line Items]
Less Than 12 Consecutive Months, Gross Unrealized Losses	$ (65)	$ (255)
Less Than 12 Consecutive Months, Fair Value	1,865	4,523
12 Consecutive Months or Longer, Gross Unrealized Losses	(2,480)	(4,158)
12 Consecutive Months or Longer, Fair Value	13,811	19,768
Fannie Mae [Member]
Schedule of Available-for-sale Securities [Line Items]
Less Than 12 Consecutive Months, Gross Unrealized Losses	(1)	(4)
Less Than 12 Consecutive Months, Fair Value	259	519
12 Consecutive Months or Longer, Gross Unrealized Losses	(10)	(13)
12 Consecutive Months or Longer, Fair Value	334	208
Alt-A private-label securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Less Than 12 Consecutive Months, Gross Unrealized Losses	(15)	(133)
Less Than 12 Consecutive Months, Fair Value	748	1,414
12 Consecutive Months or Longer, Gross Unrealized Losses	(905)	(1,731)
12 Consecutive Months or Longer, Fair Value	5,159	6,525
Subprime private-label securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Less Than 12 Consecutive Months, Gross Unrealized Losses	(17)	(73)
Less Than 12 Consecutive Months, Fair Value	340	471
12 Consecutive Months or Longer, Gross Unrealized Losses	(1,153)	(1,914)
12 Consecutive Months or Longer, Fair Value	5,583	6,686
CMBS [Member]
Schedule of Available-for-sale Securities [Line Items]
Less Than 12 Consecutive Months, Gross Unrealized Losses	0	(20)
Less Than 12 Consecutive Months, Fair Value	0	1,458
12 Consecutive Months or Longer, Gross Unrealized Losses	(20)	(84)
12 Consecutive Months or Longer, Fair Value	272	2,790
Mortgage revenue bonds [Member]
Schedule of Available-for-sale Securities [Line Items]
Less Than 12 Consecutive Months, Gross Unrealized Losses	(29)	(4)
Less Than 12 Consecutive Months, Fair Value	411	114
12 Consecutive Months or Longer, Gross Unrealized Losses	(78)	(116)
12 Consecutive Months or Longer, Fair Value	908	1,971
Other mortgage-related securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Less Than 12 Consecutive Months, Gross Unrealized Losses	(3)	(21)
Less Than 12 Consecutive Months, Fair Value	107	547
12 Consecutive Months or Longer, Gross Unrealized Losses	(314)	(300)
12 Consecutive Months or Longer, Fair Value	$ 1,555	$ 1,588