Risk Management Fair values of price risk management assets and liabilities (Details) - USD ($) $ in Millions	Jun. 30, 2019	Dec. 31, 2018
Current Assets, Commodity Contracts:
Electricity	$ 15	$ 11
Natural gas	5	7
Total current derivative assets	20	18
Noncurrent Assets, Commodity Contracts: [Abstract]
Total derivative assets not designated as hedging instruments	23	20
Total derivative assets	23	20
Current Liabilities, Commodity Contracts: [Abstract]
Electricity	14	16
Natural gas	26	35
Total current derivative liabilities	40	51
Noncurrent Liabilities, Commodity Contracts: [Abstract]
Electricity	63	78
Natural gas	13	23
Total noncurrent derivative liabilities	76	101
Total derivative liabilities not designated as hedging instruments	116	152
Total derivative liabilities	116	152
Commodity Contract Asset, Noncurrent, Electricity	0	1
Commodity Contract Asset, Noncurrent, Natural Gas	3	1
Derivative Asset, Noncurrent	$ 3	$ 2