Fair Value Measurements and Credit Concentration (Net Asset and Liability Measured on Recurring Basis Level 3 Gain Loss Included in Earnings and Other Recurring Numeric Data) (Details) (Recurring [Member], Level 3 [Member], USD $) In Millions, unless otherwise specified	3 Months Ended
	Mar. 31, 2015	Mar. 31, 2014
Net gains and losses on assets and liabilities [Abstract]
Total gains (losses) included in earnings	$ (17)	$ (135)
Energy Commodities, Net [Member] | Unregulated Wholesale Energy [Member]
Net gains and losses on assets and liabilities [Abstract]
Total gains (losses) included in earnings	21	(89)
Change in unrealized gains (losses) relating to positions still held at the reporting date	25	(13)
Energy Commodities, Net [Member] | Unregulated Retail Energy [Member]
Net gains and losses on assets and liabilities [Abstract]
Total gains (losses) included in earnings	(40)	(63)
Change in unrealized gains (losses) relating to positions still held at the reporting date	(9)	(33)
Energy Commodities, Net [Member] | Energy Purchases [Member]
Net gains and losses on assets and liabilities [Abstract]
Total gains (losses) included in earnings	2	17
Change in unrealized gains (losses) relating to positions still held at the reporting date	$ 1	$ 1