Commitments And Contingencies (Third-Party Power Purchases) (Detail) (USD $) In Millions, unless otherwise specified	6 Months Ended
	Jun. 30, 2012	Jun. 30, 2011
Long-term Purchase Commitment [Line Items]
2012	$ 1,337
2013	2,987
2014	3,142
2015	3,105
2016	2,998
Thereafter	32,911
Total	46,480
Cost incurred by the Utility under power purchase agreements	978.0	1,100.0
Total minimum lease payments terminated	136
Capital leases, future minimum lease payments	$ 125
Minimum [Member]
Long-term Purchase Commitment [Line Items]
Long-term agreements range, years	10
Maximum [Member]
Long-term Purchase Commitment [Line Items]
Long-term agreements range, years	25