Investments in Securities Available-for-sale Securities 3 (Details) - USD ($) $ in Millions	Jun. 30, 2015	Dec. 31, 2014
Schedule of Available-for-sale Securities [Line Items]
Less than 12 Consecutive Months, Gross Unrealized Losses	$ (20)	$ (4)
Less than 12 Consecutive Months, Fair Value	906	339
12 Consecutive Months or Longer, Gross Unrealized Losses	(29)	(203)
12 Consecutive Months or Longer, Fair Value	708	2,375
Fannie Mae [Member]
Schedule of Available-for-sale Securities [Line Items]
Less than 12 Consecutive Months, Gross Unrealized Losses	(5)	0
Less than 12 Consecutive Months, Fair Value	332	113
12 Consecutive Months or Longer, Gross Unrealized Losses	(17)	(19)
12 Consecutive Months or Longer, Fair Value	557	627
Alt-A private-label securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Less than 12 Consecutive Months, Gross Unrealized Losses	0	(2)
Less than 12 Consecutive Months, Fair Value	33	171
12 Consecutive Months or Longer, Gross Unrealized Losses	(3)	(13)
12 Consecutive Months or Longer, Fair Value	29	112
Subprime private-label securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Less than 12 Consecutive Months, Gross Unrealized Losses	(1)	0
Less than 12 Consecutive Months, Fair Value	53	0
12 Consecutive Months or Longer, Gross Unrealized Losses	(5)	(24)
12 Consecutive Months or Longer, Fair Value	102	460
Mortgage revenue bonds [Member]
Schedule of Available-for-sale Securities [Line Items]
Less than 12 Consecutive Months, Gross Unrealized Losses	(14)	(2)
Less than 12 Consecutive Months, Fair Value	488	47
12 Consecutive Months or Longer, Gross Unrealized Losses	(4)	(11)
12 Consecutive Months or Longer, Fair Value	$ 20	155
Other mortgage-related securities [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		8
12 Consecutive Months or Longer, Gross Unrealized Losses		(136)
12 Consecutive Months or Longer, Fair Value		$ 1,021