Price Risk Management Fair values of price risk management assets and liabilities (Details) - USD ($) $ in Millions	Sep. 30, 2017	Dec. 31, 2016
Current Assets, Commodity Contracts:
Electricity	$ 3	$ 6
Natural gas	1	12
Total current derivative assets	4	18
Noncurrent Assets, Commodity Contracts: [Abstract]
Commodity Contract Asset, Noncurrent, Electricity	0	1
Commodity Contract Asset, Noncurrent, Natural Gas	0	4
Total noncurrent derivative assets	0	5
Total derivative assets not designated as hedging instruments	4	23
Total derivative assets	4	23
Current Liabilities, Commodity Contracts: [Abstract]
Electricity	11	12
Natural gas	32	32
Total current derivative liabilities	43	44
Noncurrent Liabilities, Commodity Contracts: [Abstract]
Electricity	132	106
Natural gas	18	19
Total noncurrent derivative liabilities	150	125
Total derivative liabilities not designated as hedging instruments	193	169
Total derivative liabilities	$ 193	$ 169