Risk Management Fair values of price risk management assets and liabilities (Details) - USD ($) $ in Millions	Jun. 30, 2018	Dec. 31, 2017
Current Assets, Commodity Contracts:
Electricity	$ 3	$ 3
Natural gas	1	3
Total current derivative assets	4	6
Noncurrent Assets, Commodity Contracts: [Abstract]
Total derivative assets not designated as hedging instruments	4	6
Total derivative assets	4	6
Current Liabilities, Commodity Contracts: [Abstract]
Electricity	12	13
Natural gas	39	46
Total current derivative liabilities	51	59
Noncurrent Liabilities, Commodity Contracts: [Abstract]
Electricity	108	122
Natural gas	28	29
Total noncurrent derivative liabilities	136	151
Total derivative liabilities not designated as hedging instruments	187	210
Total derivative liabilities	$ 187	$ 210