Derivative Instruments and Hedging Activities (Intro) (Details)	3 Months Ended		6 Months Ended
	Jun. 30, 2015 USD ($) MW	Jun. 30, 2014 USD ($)	Jun. 30, 2015 USD ($) MW	Jun. 30, 2014 USD ($)	Dec. 31, 2014 USD ($)
Commodity Price Risk (Non-trading) [Abstract]
Generation fleet capacity (in MW) | MW	14,962		14,962
Commodity Price Risk Nontrading Economic Activity Numeric [Abstract]
Year of expiration of the maximum maturity date of economic activity derivative contracts			2020
Pre-tax Gains (Losses) of Economic Activity (Details) [Abstract]
Net unrealized after-tax gains (losses) expected to be reclassified into earnings within 12 months related to commodity price risk cash flow hedge contracts	$ 16,000,000		$ 16,000,000
Wholesale energy
Pre-tax Gains (Losses) of Economic Activity (Details) [Abstract]
Pre-tax gain (loss)	(65,000,000)	$ (91,000,000)	(157,000,000)	$ (880,000,000)
Retail Energy
Pre-tax Gains (Losses) of Economic Activity (Details) [Abstract]
Pre-tax gain (loss)	(9,000,000)	4,000,000	(22,000,000)	(22,000,000)
Fuel
Pre-tax Gains (Losses) of Economic Activity (Details) [Abstract]
Pre-tax gain (loss)	16,000,000	7,000,000	16,000,000	6,000,000
Energy purchases
Pre-tax Gains (Losses) of Economic Activity (Details) [Abstract]
Pre-tax gain (loss)	95,000,000	39,000,000	240,000,000	619,000,000
Credit risk
Master Netting Arrangements
Obligation to return counterparty cash collateral under master netting arrangements	10,000,000		10,000,000		$ 11,000,000
Cash collateral posted under master netting arrangements	$ 4,000,000		$ 4,000,000		$ 0
Price Risk Derivative
Pre-tax Gains (Losses) of Economic Activity (Details) [Abstract]
Net energy trading margins		$ 44,000,000		$ 58,000,000