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	3 Months Ended	12 Months Ended				51 Months Ended
	Dec. 31, 2011	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012	Dec. 31, 2015
Gain (Loss) on Derivatives [Member]
Commodity contracts:
Electricity	$ 21	$ 2	$ 11	$ 29	$ 61	$ 124
Natural gas	33	1	13	53	110	210
Net unrealized loss	$ 54	$ 3	$ 24	$ 82	$ 171	$ 334