Mortgage Loans Loans in Mortgage Portfolio (Details) (USD $) In Millions, unless otherwise specified	Sep. 30, 2013	Jun. 30, 2013	Dec. 31, 2012	Sep. 30, 2012	Jun. 30, 2012	Dec. 31, 2011
Loans in Mortgage Portfolio [Line Items]
Unpaid principal balance of mortgage loans	$ 3,029,090		$ 2,978,693
Cost basis and fair value adjustments, net	29,259		29,508
Allowance for loan losses for loans held for investment	(45,169)	(49,643)	(58,795)	(63,012)	(63,375)	(72,156)
Total mortgage loans	3,013,180		2,949,406
Single-Family [Member]
Loans in Mortgage Portfolio [Line Items]
Unpaid principal balance of mortgage loans	2,842,555		2,790,276
Allowance for loan losses for loans held for investment	(44,476)	(48,770)	(57,687)	(61,897)	(62,134)	(70,633)
Multifamily [Member]
Loans in Mortgage Portfolio [Line Items]
Unpaid principal balance of mortgage loans	186,535		188,417
Allowance for loan losses for loans held for investment	(693)	(873)	(1,108)	(1,115)	(1,241)	(1,523)
Fannie Mae [Member]
Loans in Mortgage Portfolio [Line Items]
Unpaid principal balance of mortgage loans	328,014		370,741
Cost basis and fair value adjustments, net	(13,790)		(14,805)
Allowance for loan losses for loans held for investment	(41,345)	(44,825)	(50,519)	(54,214)	(52,082)	(57,309)
Total mortgage loans	272,879		305,417
Fannie Mae [Member] | Single-Family [Member]
Loans in Mortgage Portfolio [Line Items]
Unpaid principal balance of mortgage loans	283,361		309,277
Allowance for loan losses for loans held for investment	(40,923)	(44,332)	(49,848)	(53,550)	(51,322)	(56,294)
Fannie Mae [Member] | Multifamily [Member]
Loans in Mortgage Portfolio [Line Items]
Unpaid principal balance of mortgage loans	44,653		61,464
Allowance for loan losses for loans held for investment	(422)	(493)	(671)	(664)	(760)	(1,015)
Consolidated Trusts [Member]
Loans in Mortgage Portfolio [Line Items]
Unpaid principal balance of mortgage loans	2,701,076		2,607,952
Cost basis and fair value adjustments, net	43,049		44,313
Allowance for loan losses for loans held for investment	(3,824)	(4,818)	(8,276)	(8,798)	(11,293)	(14,847)
Total mortgage loans	2,740,301		2,643,989
Consolidated Trusts [Member] | Single-Family [Member]
Loans in Mortgage Portfolio [Line Items]
Unpaid principal balance of mortgage loans	2,559,194		2,480,999
Allowance for loan losses for loans held for investment	(3,553)	(4,438)	(7,839)	(8,347)	(10,812)	(14,339)
Consolidated Trusts [Member] | Multifamily [Member]
Loans in Mortgage Portfolio [Line Items]
Unpaid principal balance of mortgage loans	141,882		126,953
Allowance for loan losses for loans held for investment	$ (271)	$ (380)	$ (437)	$ (451)	$ (481)	$ (508)