Fair values of price risk management assets and liabilities (Details) - USD ($) $ in Millions	Dec. 31, 2015	Dec. 31, 2014
Current Assets, Commodity Contracts:
Electricity	$ 7	$ 4
Natural Gas	3	2
Total current derivative assets	10	6
Noncurrent Assets, Commodity Contracts: [Abstract]
Commodity Contract Asset, Noncurrent, Electricity	0	1
Derivative Asset, Noncurrent	0	1
Total derivative assets not designated as hedging instruments	10	7
Total derivative assets	10	7
Current Liabilities, Commodity Contracts: [Abstract]
Electricity	36	54
Natural Gas	94	52
Total current derivative liabilities	130	106
Noncurrent Liabilities, Commodity Contracts: [Abstract]
Electricity	97	58
Natural Gas	64	64
Total noncurrent derivative liabilities	161	122
Total derivative liabilities not designated as hedging instruments	291	228
Total derivative liabilities	$ 291	$ 228