Investments in Securities Available-for-sale Securities 3 (Details) (USD $) In Millions, unless otherwise specified	Dec. 31, 2012	Dec. 31, 2011
Schedule of Available-for-sale Securities [Line Items]
Less Than 12 Consecutive Months, Gross Unrealized Losses	$ (43)	$ (255)
Less Than 12 Consecutive Months, Fair Value	1,605	4,523
12 Consecutive Months or Longer, Gross Unrealized Losses	(2,006)	(4,158)
12 Consecutive Months or Longer, Fair Value	12,862	19,768
Fannie Mae [Member]
Schedule of Available-for-sale Securities [Line Items]
Less Than 12 Consecutive Months, Gross Unrealized Losses	(5)	(4)
Less Than 12 Consecutive Months, Fair Value	599	519
12 Consecutive Months or Longer, Gross Unrealized Losses	(11)	(13)
12 Consecutive Months or Longer, Fair Value	372	208
Alt-A private-label securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Less Than 12 Consecutive Months, Gross Unrealized Losses	(18)	(133)
Less Than 12 Consecutive Months, Fair Value	541	1,414
12 Consecutive Months or Longer, Gross Unrealized Losses	(715)	(1,731)
12 Consecutive Months or Longer, Fair Value	4,465	6,525
Subprime private-label securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Less Than 12 Consecutive Months, Gross Unrealized Losses	(14)	(73)
Less Than 12 Consecutive Months, Fair Value	243	471
12 Consecutive Months or Longer, Gross Unrealized Losses	(893)	(1,914)
12 Consecutive Months or Longer, Fair Value	5,058	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	(11)	(84)
12 Consecutive Months or Longer, Fair Value	240	2,790
Mortgage revenue bonds [Member]
Schedule of Available-for-sale Securities [Line Items]
Less Than 12 Consecutive Months, Gross Unrealized Losses	(3)	(4)
Less Than 12 Consecutive Months, Fair Value	127	114
12 Consecutive Months or Longer, Gross Unrealized Losses	(94)	(116)
12 Consecutive Months or Longer, Fair Value	1,198	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	95	547
12 Consecutive Months or Longer, Gross Unrealized Losses	(282)	(300)
12 Consecutive Months or Longer, Fair Value	$ 1,529	$ 1,588