Fair Value (Details 7) (USD $) In Millions, unless otherwise specified	Jun. 30, 2012	Dec. 31, 2011
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Carrying value at estimated fair value	$ 57,201	$ 56,915
Variable interest entities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Unpaid principal balance	2,814	3,019
Excess estimated fair value over unpaid principal balance	118	119
Carrying value at estimated fair value	2,981	3,187
Recurring [Member] | Variable interest entities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Carrying value at estimated fair value	$ 2,932	$ 3,138