Price Risk Management Fair values of price risk management assets and liabilities (Details) - USD ($) $ in Millions	Sep. 30, 2015	Dec. 31, 2014
Current Assets, Commodity Contracts:
Electricity	$ 5	$ 4
Natural gas	2	2
Total current derivative assets	7	6
Noncurrent Assets, Commodity Contracts: [Abstract]
Electricity	0	1
Total noncurrent derivative assets	0	1
Total derivative assets not designated as hedging instruments	7	7
Total derivative assets	7	7
Current Liabilities, Commodity Contracts: [Abstract]
Electricity	38	54
Natural gas	77	52
Total current derivative liabilities	115	106
Noncurrent Liabilities, Commodity Contracts: [Abstract]
Electricity	110	58
Natural gas	74	64
Total noncurrent derivative liabilities	184	122
Total derivative liabilities not designated as hedging instruments	299	228
Total derivative liabilities	$ 299	$ 228