Price Risk Management Future Year Net Unrealized Gain/Loss Recorded at Balance Sheet Date Expected to Become Realized (Details) (Unrealized Gain Loss On Derivatives [Member], USD $) In Millions, unless otherwise specified	3 Months Ended	12 Months Ended				51 Months Ended
	Dec. 31, 2013	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2017
Unrealized Gain Loss On Derivatives [Member]
Commodity contracts:
Electricity	$ 8	$ 0	$ 7	$ 23	$ 40	$ 78
Natural gas	21	4	11	11	34	81
Net unrealized loss	$ 29	$ 4	$ 18	$ 34	$ 74	$ 159