Price Risk Management Fair values of price risk management assets and liabilities (Details) - USD ($) $ in Millions	Mar. 31, 2016	Dec. 31, 2015
Current Assets, Commodity Contracts:
Electricity	$ 11	$ 7
Natural gas	7	3
Total current derivative assets	18	10
Noncurrent Assets, Commodity Contracts: [Abstract]
Commodity Contract Asset, Noncurrent, Natural Gas	2	0
Total noncurrent derivative assets	2	0
Total derivative assets not designated as hedging instruments	20	10
Total derivative assets	20	10
Current Liabilities, Commodity Contracts: [Abstract]
Electricity	37	36
Natural gas	105	94
Total current derivative liabilities	142	130
Noncurrent Liabilities, Commodity Contracts: [Abstract]
Electricity	109	97
Natural gas	52	64
Total noncurrent derivative liabilities	161	161
Total derivative liabilities not designated as hedging instruments	303	291
Total derivative liabilities	$ 303	$ 291