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	9 Months Ended	12 Months Ended			45 Months Ended
	Dec. 31, 2012	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2015
Loss on Derivatives [Member]
Commodity contracts:
Electricity	$ 63	$ 11	$ 25	$ 60	$ 159
Natural gas	111	7	30	83	231
Net unrealized loss	$ 174	$ 18	$ 55	$ 143	$ 390