Fair Value Measurements (Quantitative Information About Level 3 Fair Value Measurements) (Details) (USD $) In Millions, unless otherwise specified						12 Months Ended				12 Months Ended
	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2012 Level 3	Dec. 31, 2011 Level 3	Dec. 31, 2012 Level 3 Electricity Congestion Contracts	Dec. 31, 2012 Level 3 Electricity Congestion Contracts Latest Auction Pricing Congestion Prices Minimum	Dec. 31, 2012 Level 3 Electricity Congestion Contracts Latest Auction Pricing Congestion Prices Maximum	Dec. 31, 2012 Level 3 Electricity Congestion Contracts Latest Auction Pricing Congestion Prices Weighted Average	Dec. 31, 2012 Level 3 Electricity Power Contracts	Dec. 31, 2012 Level 3 Electricity Power Contracts Discounted Cash Flow Power Prices Minimum	Dec. 31, 2012 Level 3 Electricity Power Contracts Discounted Cash Flow Power Prices Maximum	Dec. 31, 2012 Level 3 Electricity Power Contracts Discounted Cash Flow Power Prices Weighted Average	Dec. 31, 2012 Level 3 Netting
Quantitative Information About Level 3 Measurements [Line Items]
Fair Value, Assets	$ 705	$ 1,281	$ 52	$ 95	$ 71				$ 2				$ (21)
Fair Value, Liabilities	$ 118	$ 91	$ 1	$ 12	$ 20				$ 2				$ (21)
Fair Value inputs, price level						(8.93)	18.03	0.19		22.54	48.85	39.62