Price Risk Management Net realized and unrealized gains and losses on derivative transactions (Details) (USD $) In Millions, unless otherwise specified	12 Months Ended
	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
Derivative [Line Items]
Net gain or loss recognized in the statement of income offset by regulatory accounting	$ 42	$ 192	$ 258
Commodity contracts: [Abstract]
Commodity Contracts, Electricity	56	117	127
Contracts, Natural Gas	$ 19	$ 98	$ 192