Price Risk Management Future Year Net Unrealized Gain/Loss Recorded at Balance Sheet Date Expected to Become Realized (Details) (Loss on Derivatives [Member], USD $) In Millions, unless otherwise specified	4 Months Ended	12 Months Ended				52 Months Ended
	Dec. 31, 2012	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2016
Loss on Derivatives [Member]
Commodity contracts:
Electricity	$ 13	$ 0	$ 11	$ 24	$ 46	$ 94
Natural gas	35	(2)	4	24	72	133
Net unrealized loss	$ 48	$ (2)	$ 15	$ 48	$ 118	$ 227