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	3 Months Ended
	Mar. 31, 2012	Mar. 31, 2011
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	4.44%
Weighted average maturity for debt securities held by NDT funds (in years)	8.30
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	$ 4	$ 1
Change in unrealized gains (losses) relating to positions still held at the reporting date	(18)	0
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	16	1
Change in unrealized gains (losses) relating to positions still held at the reporting date	46	1
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)	(5)
Change in unrealized gains (losses) relating to positions still held at the reporting date	(1)	(1)
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	(1)	4
Change in unrealized gains (losses) relating to positions still held at the reporting date	$ (5)	$ 19