Commitments And Contingencies (Third-Party Power Purchases) (Detail) (USD $) In Millions, unless otherwise specified	6 Months Ended
	Jun. 30, 2013
Long-term Purchase Commitment [Line Items]
Quantity of Long-Term Power Purchase Renewable Energy Agreements	7
Total	$ 1,100
Minimum [Member]
Long-term Purchase Commitment [Line Items]
Long-term agreements range, years	5
Maximum [Member]
Long-term Purchase Commitment [Line Items]
Long-term agreements range, years	25