Fair values of price risk management assets and liabilities (Details) - USD ($) $ in Millions	Dec. 31, 2016	Dec. 31, 2015
Current Assets, Commodity Contracts:
Electricity	$ 6	$ 7
Natural Gas	12	3
Total current derivative assets	18	10
Noncurrent Assets, Commodity Contracts: [Abstract]
Commodity Contract Asset, Noncurrent, Electricity	1	0
Commodity Contract Asset, Noncurrent, Natural Gas	4	0
Derivative Asset, Noncurrent	5	0
Total derivative assets not designated as hedging instruments	23	10
Total derivative assets	23	10
Current Liabilities, Commodity Contracts: [Abstract]
Electricity	12	36
Natural Gas	32	94
Total current derivative liabilities	44	130
Noncurrent Liabilities, Commodity Contracts: [Abstract]
Electricity	106	97
Natural Gas	19	64
Total noncurrent derivative liabilities	125	161
Total derivative liabilities not designated as hedging instruments	169	291
Total derivative liabilities	$ 169	$ 291