Fair Value Fair Value Option (Details) (USD $) In Millions, unless otherwise specified	Dec. 31, 2013		Dec. 31, 2012
Fair Value, Option, Quantitative Disclosures [Line Items]
Fair value of nonaccrual loans	$ 196		$ 273
Difference between unpaid principal balance and the fair value of nonaccrual loans	74		189
Fair value of loans that are 90 days past due	288		386
Difference between unpaid principal balance and the fair value of these 90 days or more days past due loans	75		201
Consolidated Trusts [Member]
Fair Value, Option, Quantitative Disclosures [Line Items]
Mortgage loans held for investment	14,268		10,800
Long-term debt	14,976		11,647
Fannie Mae [Member]
Fair Value, Option, Quantitative Disclosures [Line Items]
Long-term debt	1,308		793
Interest-only debt instruments [Member]
Fair Value, Option, Quantitative Disclosures [Line Items]
Long-term debt	85		100
Loans [Member] | Consolidated Trusts [Member]
Fair Value, Option, Quantitative Disclosures [Line Items]
Mortgage loans held for investment	14,268	[1]	10,800	[1]
Loans, Unpaid principal balance	14,440	[1]	10,657	[1]
Long-term Debt [Member] | Consolidated Trusts [Member]
Fair Value, Option, Quantitative Disclosures [Line Items]
Long-term debt	14,976	[2]	11,647	[2]
Long-Term Debt, Unpaid principal balance	13,988	[2]	10,803	[2]
Long-term Debt [Member] | Fannie Mae [Member]
Fair Value, Option, Quantitative Disclosures [Line Items]
Long-term debt	1,308		793
Long-Term Debt, Unpaid principal balance	$ 1,290		$ 674

[1] Includes nonaccrual loans with a fair value of $196 million and $273 million as of December 31, 2013 and 2012, respectively. The difference between unpaid principal balance and the fair value of these nonaccrual loans as of December 31, 2013 and 2012 is $74 million and $189 million, respectively. Includes loans that are 90 days or more past due with a fair value of $288 million and $386 million as of December 31, 2013 and 2012, respectively. The difference between unpaid principal balance and the fair value of these 90 or more days past due loans as of December 31, 2013 and 2012 is $75 million and $201 million, respectively.

[2] Includes interest-only debt instruments with no unpaid principal balance and a fair value of $85 million and $100 million as of December 31, 2013 and 2012, respectively.