Disclosures About Offsetting Assets and Liabilities - Offsetting Assets (Details) (USD $) In Millions, unless otherwise specified	Mar. 31, 2013		Dec. 31, 2012
Offsetting [Abstract]
Derivatives, Gross Amounts of Recognized Assets	$ 1,310		$ 1,683
Derivatives, Gross Amounts Offset in the Balance Sheet	1,179	[1]	1,683	[1]
Derivatives, Net Amounts of Assets Presented in the Balance Sheet	131		0
Derivatives, Gross Amounts Not Offset in the Balance Sheet, Financial Instruments	0		0
Derivatives, Gross Amounts Not Offset in the Balance Sheet, Cash Collateral Received	0		0
Derivatives, Net Amount	131		0
Securities purchased under agreements to resell, Gross Amounts of Recognized Assets	3,275		2,160
Securities purchased under agreements to resell, Gross Amounts Offset in the Balance Sheet	0	[1]	0	[1]
Securities purchased under agreements to resell, Net Amount of Assets Presented in the Balance Sheet	3,275		2,160
Securities purchased under agreements to resell, Gross Amounts Not Offset in the Balance Sheet, Financial Instruments	0		0
Securities purchased under agreements to resell, Gross Amounts Not Offset in the Balance Sheet, Cash Collateral Received	0		0
Securites purchased under agreements to resell, Net Amount	3,275		2,160
Total, Gross Amounts of Recognized Assets	4,585		3,843
Total, Gross Amounts Offset in the Balance Sheet	1,179	[1]	1,683	[1]
Total, Net Amounts of Assets Presented in the Balance Sheet	3,406		2,160
Total, Gross AMounts Not Offset in the Balance Sheet, Financial Instruments	0		0
Total, Gross Amounts Not Offset in the Balance Sheet, Cash Collateral Received	0		0
Total, Gross Amounts Not Offset in the Balance Sheet, Net Amount	3,406		2,160
Collateral received, offset amount	$ 347		$ 75

[1] Gross amounts offset in the balance sheet includes collateral received as of March 31, 2013 and December 31, 2012 of $347 million and $75 million, respectively.