Investments in Securities Available-for-sale Securities 3 (Details) (USD $) In Millions, unless otherwise specified	Sep. 30, 2013	Dec. 31, 2012
Schedule of Available-for-sale Securities [Line Items]
Less than 12 Consecutive Months, Gross Unrealized Losses	$ (247)	$ (43)
Less Than 12 Consecutive Months, Fair Value	3,909	1,605
12 Consecutive Months or Longer, Gross Unrealized Losses	(673)	(2,006)
12 Consecutive Months or Longer, Fair Value	4,520	12,862
Fannie Mae [Member]
Schedule of Available-for-sale Securities [Line Items]
Less than 12 Consecutive Months, Gross Unrealized Losses	(44)	(5)
Less Than 12 Consecutive Months, Fair Value	991	599
12 Consecutive Months or Longer, Gross Unrealized Losses	(3)	(11)
12 Consecutive Months or Longer, Fair Value	137	372
Alt-A private-label securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Less than 12 Consecutive Months, Gross Unrealized Losses	(16)	(18)
Less Than 12 Consecutive Months, Fair Value	436	541
12 Consecutive Months or Longer, Gross Unrealized Losses	(64)	(715)
12 Consecutive Months or Longer, Fair Value	618	4,465
Subprime private-label securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Less than 12 Consecutive Months, Gross Unrealized Losses	(64)	(14)
Less Than 12 Consecutive Months, Fair Value	851	243
12 Consecutive Months or Longer, Gross Unrealized Losses	(154)	(893)
12 Consecutive Months or Longer, Fair Value	1,581	5,058
CMBS [Member]
Schedule of Available-for-sale Securities [Line Items]
Less than 12 Consecutive Months, Gross Unrealized Losses		0
Less Than 12 Consecutive Months, Fair Value		0
12 Consecutive Months or Longer, Gross Unrealized Losses		(11)
12 Consecutive Months or Longer, Fair Value		240
Mortgage revenue bonds [Member]
Schedule of Available-for-sale Securities [Line Items]
Less than 12 Consecutive Months, Gross Unrealized Losses	(122)	(3)
Less Than 12 Consecutive Months, Fair Value	1,580	127
12 Consecutive Months or Longer, Gross Unrealized Losses	(234)	(94)
12 Consecutive Months or Longer, Fair Value	1,008	1,198
Other mortgage-related securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Less than 12 Consecutive Months, Gross Unrealized Losses	(1)	(3)
Less Than 12 Consecutive Months, Fair Value	51	95
12 Consecutive Months or Longer, Gross Unrealized Losses	(218)	(282)
12 Consecutive Months or Longer, Fair Value	$ 1,176	$ 1,529