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, 2012	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2015
Loss on Derivatives [Member]
Commodity contracts:
Electricity	$ 24	$ 10	$ 22	$ 55	$ 111
Natural gas	80	7	28	79	194
Net unrealized loss	$ 104	$ 17	$ 50	$ 134	$ 305