Risk Management Fair values of price risk management assets and liabilities (Details) - USD ($) $ in Millions	Mar. 31, 2019	Dec. 31, 2018
Current Assets, Commodity Contracts:
Electricity	$ 17	$ 11
Natural gas	16	7
Total current derivative assets	33	18
Noncurrent Assets, Commodity Contracts: [Abstract]
Total derivative assets not designated as hedging instruments	37	20
Total derivative assets	37	20
Current Liabilities, Commodity Contracts: [Abstract]
Electricity	12	16
Natural gas	20	35
Total current derivative liabilities	32	51
Noncurrent Liabilities, Commodity Contracts: [Abstract]
Electricity	63	78
Natural gas	15	23
Total noncurrent derivative liabilities	78	101
Total derivative liabilities not designated as hedging instruments	110	152
Total derivative liabilities	110	152
Commodity Contract Asset, Noncurrent, Electricity	1	1
Commodity Contract Asset, Noncurrent, Natural Gas	3	1
Derivative Asset, Noncurrent	$ 4	$ 2