Price Risk Management Fair values of price risk management assets and liabilities (Details) (USD $) In Millions, unless otherwise specified	Sep. 30, 2012	Dec. 31, 2011
Current Assets, Commodity Contracts:
Electricity	$ 2	$ 2
Natural gas	3	17
Total current derivative assets	5	19
Noncurrent Assets, Commodity Contracts: [Abstract]
Commodity Contracts-Natural gas	5	0
Total derivative assets not designated as hedging instruments	10	19
Total derivative assets	10	19
Current Liabilities, Commodity Contracts: [Abstract]
Electricity	54	66
Natural gas	93	150
Total current derivative liabilities	147	216
Noncurrent Liabilities, Commodity Contracts: [Abstract]
Electricity	42	71
Natural gas	48	101
Total noncurrent derivative liabilities	90	172
Total derivative liabilities not designated as hedging instruments	237	388
Total derivative liabilities	$ 237	$ 388