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) - Level 3 - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2015	Sep. 30, 2014	Sep. 30, 2015	Sep. 30, 2014
Net gains and losses on assets and liabilities [Abstract]
Total gains (losses) included in earnings	$ (7)	$ (84)	$ (148)	$ (147)
Recurring | Energy Commodities, Net | Wholesale energy
Net gains and losses on assets and liabilities [Abstract]
Total gains (losses) included in earnings	2	(102)	(108)	(133)
Change in unrealized gains (losses) relating to positions still held at the reporting date	8	6	(31)	5
Recurring | Energy Commodities, Net | Retail Energy
Net gains and losses on assets and liabilities [Abstract]
Total gains (losses) included in earnings	(1)	16	(33)	(35)
Change in unrealized gains (losses) relating to positions still held at the reporting date	12	13	9	(12)
Recurring | Energy Commodities, Net | Energy purchases
Net gains and losses on assets and liabilities [Abstract]
Total gains (losses) included in earnings	(8)	2	(7)	21
Change in unrealized gains (losses) relating to positions still held at the reporting date	$ (1)	$ 1	$ (5)	$ (3)