Concentrations of Credit Risk Financial Guarantor (Details) (USD $) In Millions, unless otherwise specified	Dec. 31, 2012	Dec. 31, 2011
Concentration Risk [Line Items]
Unpaid principal balance of guaranteed non-agency securities in our portfolio	$ 6,858	$ 7,971
Alt-A private-label securities [Member]
Concentration Risk [Line Items]
Unpaid principal balance of guaranteed non-agency securities in our portfolio	928	1,279
Subprime private-label securities [Member]
Concentration Risk [Line Items]
Unpaid principal balance of guaranteed non-agency securities in our portfolio	1,264	1,398
Mortgage revenue bonds [Member]
Concentration Risk [Line Items]
Unpaid principal balance of guaranteed non-agency securities in our portfolio	4,374	4,931
Other mortgage-related securities [Member]
Concentration Risk [Line Items]
Unpaid principal balance of guaranteed non-agency securities in our portfolio	292	317
Non-mortgage-related Securities [Member]
Concentration Risk [Line Items]
Unpaid principal balance of guaranteed non-agency securities in our portfolio	$ 0	$ 46