Price Risk Management Fair values of price risk management assets and liabilities (Details) - USD ($) $ in Millions	Jun. 30, 2016	Dec. 31, 2015
Current Assets, Commodity Contracts:
Electricity	$ 8	$ 7
Natural gas	4	3
Total current derivative assets	12	10
Noncurrent Assets, Commodity Contracts: [Abstract]
Commodity Contract Asset, Noncurrent, Natural Gas	4	0
Commodity Contract Asset, Noncurrent, Electricity	1	0
Total noncurrent derivative assets	5	0
Total derivative assets not designated as hedging instruments	17	10
Total derivative assets	17	10
Current Liabilities, Commodity Contracts: [Abstract]
Electricity	14	36
Natural gas	67	94
Total current derivative liabilities	81	130
Noncurrent Liabilities, Commodity Contracts: [Abstract]
Electricity	138	97
Natural gas	33	64
Total noncurrent derivative liabilities	171	161
Total derivative liabilities not designated as hedging instruments	252	291
Total derivative liabilities	$ 252	$ 291