Price Risk Management Fair values of price risk management assets and liabilities (Details) (USD $) In Millions	Sep. 30, 2011	Dec. 31, 2010
Current Assets, Commodity Contracts:
Electricity	$ 2	$ 4
Natural gas	7	9
Total current derivative assets	9	13
Noncurrent Assets, Commodity Contracts: [Abstract]
Electricity	0	1
Natural gas	0	2
Total noncurrent derivative assets	0	3
Total derivative assets not designated as hedging instruments	9	16
Total derivative assets	9	16
Current Liabilities, Commodity Contracts: [Abstract]
Electricity	73	77
Natural gas	123	111
Total current derivative liabilities	196	188
Noncurrent Liabilities, Commodity Contracts: [Abstract]
Electricity	53	42
Natural gas	94	146
Total noncurrent derivative liabilities	147	188
Total derivative liabilities not designated as hedging instruments	343	376
Total derivative liabilities	$ 343	$ 376