Price Risk Management Fair values of price risk management assets and liabilities (Details) (USD $) In Millions, unless otherwise specified	Mar. 31, 2013	Dec. 31, 2012
Current Assets, Commodity Contracts:
Electricity	$ 5	$ 1
Natural gas	6	3
Total current derivative assets	11	4
Noncurrent Assets, Commodity Contracts: [Abstract]
Commodity Contracts-Natural gas	1	2
Total derivative assets not designated as hedging instruments	12	6
Total derivative assets	12	6
Current Liabilities, Commodity Contracts: [Abstract]
Electricity	31	44
Natural gas	60	83
Total current derivative liabilities	91	127
Noncurrent Liabilities, Commodity Contracts: [Abstract]
Electricity	47	38
Natural gas	31	35
Total noncurrent derivative liabilities	78	73
Total derivative liabilities not designated as hedging instruments	169	200
Total derivative liabilities	$ 169	$ 200