Risk Management and Hedging Activities Risk Management and Hedging Activities - Collateral and Contingent Features (Details) (Commodity Contract [Member], USD $) In Millions, unless otherwise specified	Mar. 31, 2013	Dec. 31, 2012
Commodity Contract [Member]
Derivative [Line Items]
Derivative, Net Liability Position, Aggregate Fair Value	$ 104	$ 153
Collateral Already Posted, Aggregate Fair Value	33	56
Additional Collateral, Aggregate Fair Value	$ 55	$ 73