Price Risk Management Fair values of price risk management assets and liabilities (Details) - USD ($) $ in Millions	Jun. 30, 2017	Dec. 31, 2016
Current Assets, Commodity Contracts:
Electricity	$ 3	$ 6
Natural gas	2	12
Total current derivative assets	5	18
Noncurrent Assets, Commodity Contracts: [Abstract]
Commodity Contract Asset, Noncurrent, Natural Gas	0	4
Commodity Contract Asset, Noncurrent, Electricity	2	1
Total noncurrent derivative assets	2	5
Total derivative assets not designated as hedging instruments	7	23
Total derivative assets	7	23
Current Liabilities, Commodity Contracts: [Abstract]
Electricity	10	12
Natural gas	36	32
Total current derivative liabilities	46	44
Noncurrent Liabilities, Commodity Contracts: [Abstract]
Electricity	136	106
Natural gas	22	19
Total noncurrent derivative liabilities	158	125
Total derivative liabilities not designated as hedging instruments	204	169
Total derivative liabilities	$ 204	$ 169