Price Risk Management Fair values of price risk management assets and liabilities (Details) (USD $) In Millions, unless otherwise specified	Mar. 31, 2014	Dec. 31, 2013
Current Assets, Commodity Contracts:
Electricity	$ 4	$ 9
Natural gas	14	4
Total current derivative assets	18	13
Noncurrent Assets, Commodity Contracts: [Abstract]
Electricity	0	1
Natural gas	3	0
Total noncurrent derivative assets	3	1
Total derivative assets not designated as hedging instruments	21	14
Total derivative assets	21	14
Current Liabilities, Commodity Contracts: [Abstract]
Electricity	33	20
Natural gas	19	29
Total current derivative liabilities	52	49
Noncurrent Liabilities, Commodity Contracts: [Abstract]
Electricity	102	107
Natural gas	24	34
Total noncurrent derivative liabilities	126	141
Total derivative liabilities not designated as hedging instruments	178	190
Total derivative liabilities	$ 178	$ 190