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, 2013	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2016
Loss on Derivatives [Member]
Commodity contracts:
Electricity	$ 20	$ 5	$ 18	$ 30	$ 73
Natural gas	50	3	6	25	84
Net unrealized loss	$ 70	$ 8	$ 24	$ 55	$ 157