Price Risk Management Future year net unrealized gain/loss recorded at balance sheet date expected to become realized (Details) (Gain (Loss) on Derivatives [Member], USD $) In Millions, unless otherwise specified	12 Months Ended			36 Months Ended
	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2015
Gain (Loss) on Derivatives [Member]
Commodity contracts:
Commodity Contracts, Electricity	$ 10	$ 28	$ 43	$ 81
Commodity Contracts, Natural Gas	6	27	80	113
Net unrealized loss	$ 16	$ 55	$ 123	$ 194