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) (USD $) In Millions, unless otherwise specified	12 Months Ended
	Dec. 31, 2011	Dec. 31, 2010
Recurring Fair Value Measurements Valuation Techniques - NDT Funds (Numeric) [Abstract]
Percentage of Wilshire 5000 Index invested in large-cap stocks	70.00%
Percentage of Wilshire 5000 Index invested in mid/small-cap stocks	30.00%
Weighted average coupon of NDT funds debt securities	4.40%
Weighted average maturity for debt securities held by NDT funds	8.46
Recurring Fair Value Measurements Valuation Techniques - Auction Rate Securities (Numeric) [Abstract]
Weighted average contractual maturity of auction rate securites (in years)	24
Energy Commodities, Net [Member] | Wholesale Energy Marketing [Member]
Net gains and losses on assets and liabilities [Abstract]
Total gains (losses) included in earnings for the period	$ 0	$ 14
Change in unrealized gains (losses) relating to positions still held at the reporting date	5	6
Energy Commodities, Net [Member] | Unregulated Retail Electric and Gas [Member]
Net gains and losses on assets and liabilities [Abstract]
Total gains (losses) included in earnings for the period	32	11
Change in unrealized gains (losses) relating to positions still held at the reporting date	23	4
Energy Commodities, Net [Member] | Net Energy Trading Margins [Member]
Net gains and losses on assets and liabilities [Abstract]
Total gains (losses) included in earnings for the period	(1)	0
Change in unrealized gains (losses) relating to positions still held at the reporting date	1	0
Energy Commodities, Net [Member] | Energy Purchases [Member]
Net gains and losses on assets and liabilities [Abstract]
Total gains (losses) included in earnings for the period	(96)	(162)
Change in unrealized gains (losses) relating to positions still held at the reporting date	$ (2)	$ (119)