Investments in Securities Available-for-sale Securities 3 (Details) (USD $) In Millions, unless otherwise specified	Jun. 30, 2012	Dec. 31, 2011
Schedule of Available-for-sale Securities [Line Items]
Less Than 12 Consecutive Months, Gross Unrealized Losses	$ (82)	$ (255)
Less Than 12 Consecutive Months, Fair Value	3,270	4,523
12 Consecutive Months or Longer, Gross Unrealized Losses	(3,222)	(4,158)
12 Consecutive Months or Longer, Fair Value	15,335	19,768
Fannie Mae [Member]
Schedule of Available-for-sale Securities [Line Items]
Less Than 12 Consecutive Months, Gross Unrealized Losses	(2)	(4)
Less Than 12 Consecutive Months, Fair Value	350	519
12 Consecutive Months or Longer, Gross Unrealized Losses	(11)	(13)
12 Consecutive Months or Longer, Fair Value	248	208
Alt-A private-label securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Less Than 12 Consecutive Months, Gross Unrealized Losses	(22)	(133)
Less Than 12 Consecutive Months, Fair Value	911	1,414
12 Consecutive Months or Longer, Gross Unrealized Losses	(1,249)	(1,731)
12 Consecutive Months or Longer, Fair Value	5,945	6,525
Subprime private-label securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Less Than 12 Consecutive Months, Gross Unrealized Losses	(13)	(73)
Less Than 12 Consecutive Months, Fair Value	284	471
12 Consecutive Months or Longer, Gross Unrealized Losses	(1,503)	(1,914)
12 Consecutive Months or Longer, Fair Value	5,999	6,686
CMBS [Member]
Schedule of Available-for-sale Securities [Line Items]
Less Than 12 Consecutive Months, Gross Unrealized Losses	(5)	(20)
Less Than 12 Consecutive Months, Fair Value	854	1,458
12 Consecutive Months or Longer, Gross Unrealized Losses	(36)	(84)
12 Consecutive Months or Longer, Fair Value	526	2,790
Mortgage revenue bonds [Member]
Schedule of Available-for-sale Securities [Line Items]
Less Than 12 Consecutive Months, Gross Unrealized Losses	(31)	(4)
Less Than 12 Consecutive Months, Fair Value	513	114
12 Consecutive Months or Longer, Gross Unrealized Losses	(82)	(116)
12 Consecutive Months or Longer, Fair Value	1,085	1,971
Other mortgage-related securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Less Than 12 Consecutive Months, Gross Unrealized Losses	(9)	(21)
Less Than 12 Consecutive Months, Fair Value	358	547
12 Consecutive Months or Longer, Gross Unrealized Losses	(341)	(300)
12 Consecutive Months or Longer, Fair Value	$ 1,532	$ 1,588