Fair Value (Details 7) (USD $) In Millions, unless otherwise specified	Sep. 30, 2012	Dec. 31, 2011
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Carrying value at estimated fair value	$ 57,884	$ 56,915
Variable interest entities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Unpaid principal balance	2,752	3,019
Difference between estimated fair value and unpaid principal balance	127	119
Carrying value at estimated fair value	2,928	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	$ 2,879	$ 3,138