Offsetting Assets and Liabilities (Details) (USD $) In Millions, unless otherwise specified	Mar. 31, 2013	Dec. 31, 2012
Derivatives:
Gross Amounts of Recognized Assets	$ 3,150	$ 3,418
Gross Amounts Not Offset in the Consolidated Balance Sheets
Financial Instruments	(2,630)	(2,676)
Cash Collateral	(150)	(262)
Securities Collateral	(241)	(355)
Net Amount	129	125
OTC derivatives
Derivatives:
Gross Amounts of Recognized Assets	3,056	3,322
Gross Amounts Not Offset in the Consolidated Balance Sheets
Financial Instruments	(2,630)	(2,676)
Cash Collateral	(150)	(262)
Securities Collateral	(241)	(355)
Net Amount	35	29
Exchange-traded derivatives
Derivatives:
Gross Amounts of Recognized Assets	94	96
Gross Amounts Not Offset in the Consolidated Balance Sheets
Net Amount	$ 94	$ 96