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	6 Months Ended	12 Months Ended			42 Months Ended
	Dec. 31, 2013	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2016
Loss on Derivatives [Member]
Commodity contracts:
Electricity	$ 12	$ 6	$ 24	$ 43	$ 85
Natural gas	43	6	9	35	93
Net unrealized loss	$ 55	$ 12	$ 33	$ 78	$ 178