Risk Management and Hedging Activities Risk Management and Hedging Activities - Collateral and Contingent Features (Details) (Commodity Contract [Member], USD $) In Millions, unless otherwise specified	Dec. 31, 2012	Dec. 31, 2011
Commodity Contract [Member]
Derivative [Line Items]
Derivative, Net Liability Position, Aggregate Fair Value	$ 153	$ 378
Collateral Already Posted, Aggregate Fair Value	56	125
Additional Collateral, Aggregate Fair Value	$ 73	$ 155