Price Risk Management Fair values of price risk management assets and liabilities (Details) - USD ($) $ in Millions	Sep. 30, 2016	Dec. 31, 2015
Current Assets, Commodity Contracts:
Electricity	$ 2	$ 7
Natural gas	3	3
Total current derivative assets	5	10
Noncurrent Assets, Commodity Contracts: [Abstract]
Commodity Contract Asset, Noncurrent, Natural Gas	2	0
Commodity Contract Asset, Noncurrent, Electricity	0	0
Total noncurrent derivative assets	2	0
Total derivative assets not designated as hedging instruments	7	10
Total derivative assets	7	10
Current Liabilities, Commodity Contracts: [Abstract]
Electricity	11	36
Natural gas	55	94
Total current derivative liabilities	66	130
Noncurrent Liabilities, Commodity Contracts: [Abstract]
Electricity	132	97
Natural gas	31	64
Total noncurrent derivative liabilities	163	161
Total derivative liabilities not designated as hedging instruments	229	291
Total derivative liabilities	$ 229	$ 291