Commodity And Other Derivative Contractual Assets And Liabilities (Credit Risk-Related Contingent Features of Derivatives) (Details) (USD $) In Millions, unless otherwise specified	Sep. 30, 2013	Dec. 31, 2012
Credit Derivatives [Line Items]
Derivative, Net Liability Position, Aggregate Fair Value	$ 1,232	$ 2,208
Credit risk derivative with contingent feature [Member]
Credit Derivatives [Line Items]
Liquidity exposure associated with liabilities reduced by cash and letter of credit postings with the counterparties	9	12
Assets Needed for Immediate Settlement, Aggregate Fair Value	8
Derivative, Net Liability Position, Aggregate Fair Value	40	58
Cross-default credit derivative [Member]
Credit Derivatives [Line Items]
Assets Needed for Immediate Settlement, Aggregate Fair Value	1,087	1,122
Derivative, Net Liability Position, Aggregate Fair Value	$ 1,192	$ 2,150