Price Risk Management Fair values of price risk management assets and liabilities (Details) (USD $) In Millions, unless otherwise specified	Mar. 31, 2012	Dec. 31, 2011
Current Assets, Commodity Contracts:
Electricity	$ 7	$ 2
Natural gas	18	17
Total current derivative assets	25	19
Total derivative assets not designated as hedging instruments	25	19
Total derivative assets	25	19
Current Liabilities, Commodity Contracts: [Abstract]
Electricity	90	66
Natural gas	152	150
Total current derivative liabilities	242	216
Noncurrent Liabilities, Commodity Contracts: [Abstract]
Electricity	76	71
Natural gas	97	101
Total noncurrent derivative liabilities	173	172
Total derivative liabilities not designated as hedging instruments	415	388
Total derivative liabilities	$ 415	$ 388