Price Risk Management Fair values of price risk management assets and liabilities (Details) (USD $) In Millions, unless otherwise specified	Jun. 30, 2013	Dec. 31, 2012
Current Assets, Commodity Contracts:
Electricity	$ 2	$ 1
Natural gas	1	3
Total current derivative assets	3	4
Noncurrent Assets, Commodity Contracts: [Abstract]
Commodity Contracts-Natural gas	0	2
Total derivative assets not designated as hedging instruments	3	6
Total derivative assets	3	6
Current Liabilities, Commodity Contracts: [Abstract]
Electricity	43	44
Natural gas	60	83
Total current derivative liabilities	103	127
Noncurrent Liabilities, Commodity Contracts: [Abstract]
Electricity	44	38
Natural gas	34	35
Total noncurrent derivative liabilities	78	73
Total derivative liabilities not designated as hedging instruments	181	200
Total derivative liabilities	$ 181	$ 200