Price Risk Management Fair values of price risk management assets and liabilities (Details) (USD $) In Millions, unless otherwise specified	Sep. 30, 2013	Dec. 31, 2012
Current Assets, Commodity Contracts:
Electricity	$ 0	$ 1
Natural gas	1	3
Total current derivative assets	1	4
Noncurrent Assets, Commodity Contracts: [Abstract]
Commodity Contracts-Natural gas	0	2
Total derivative assets not designated as hedging instruments	1	6
Total derivative assets	1	6
Current Liabilities, Commodity Contracts: [Abstract]
Electricity	42	44
Natural gas	47	83
Total current derivative liabilities	89	127
Noncurrent Liabilities, Commodity Contracts: [Abstract]
Electricity	36	38
Natural gas	35	35
Total noncurrent derivative liabilities	71	73
Total derivative liabilities not designated as hedging instruments	160	200
Total derivative liabilities	$ 160	$ 200