Commitments And Contingencies (Narrative) (Detail) (USD $)	1 Months Ended		9 Months Ended
	Mar. 31, 2007 Plaintiff	May 31, 2004	Sep. 30, 2012 GeneratingPlant Plant	Jun. 30, 2005
Loss Contingencies [Line Items]
Investigation review bids maximum (per MW)		$ 250
Number of families that own property near the holding ponds who filed a complaint	2
Owners percentage interest	15.00%
Accrual for environmental loss contingencies				$ 500,000
Number of hydroelectric plants			6
Number of years, license issued			50 years
Number of diversion tunnels			2