Fair Value Measurements (Quantitative Information About Level 3 Fair Value Measurements) (Details) (USD $) In Millions, unless otherwise specified	3 Months Ended	6 Months Ended
	Dec. 31, 2012	Jun. 30, 2013
Quantitative Information About Level 3 Measurements [Line Items]
Fair Value, Assets	$ 705	$ 919
Fair Value, Liabilities	118	68
Level 3
Quantitative Information About Level 3 Measurements [Line Items]
Fair Value, Assets	52	32
Fair Value, Liabilities	1	6
Level 3 | Electricity Congestion Contracts
Quantitative Information About Level 3 Measurements [Line Items]
Fair Value, Assets	71	50
Fair Value, Liabilities	20	18
Level 3 | Electricity Congestion Contracts | Latest Auction Pricing | Congestion Prices | Minimum
Quantitative Information About Level 3 Measurements [Line Items]
Fair Value inputs, price level	(8.93)	(8.94)
Level 3 | Electricity Congestion Contracts | Latest Auction Pricing | Congestion Prices | Maximum
Quantitative Information About Level 3 Measurements [Line Items]
Fair Value inputs, price level	18.03	13.88
Level 3 | Electricity Congestion Contracts | Latest Auction Pricing | Congestion Prices | Weighted Average
Quantitative Information About Level 3 Measurements [Line Items]
Fair Value inputs, price level	0.19	0.11
Level 3 | Electricity Power Contracts
Quantitative Information About Level 3 Measurements [Line Items]
Fair Value, Assets	2	14
Fair Value, Liabilities	2	20
Level 3 | Electricity Power Contracts | Discounted Cash Flow | Power Prices | Minimum
Quantitative Information About Level 3 Measurements [Line Items]
Fair Value inputs, price level	22.54	34.24
Level 3 | Electricity Power Contracts | Discounted Cash Flow | Power Prices | Maximum
Quantitative Information About Level 3 Measurements [Line Items]
Fair Value inputs, price level	48.85	73.00
Level 3 | Electricity Power Contracts | Discounted Cash Flow | Power Prices | Weighted Average
Quantitative Information About Level 3 Measurements [Line Items]
Fair Value inputs, price level	39.62	41.79
Level 3 | Netting
Quantitative Information About Level 3 Measurements [Line Items]
Fair Value, Assets	(21)	(32)
Fair Value, Liabilities	$ (21)	$ (32)