Components of Electricity Revenues and Cost of Revenues (Detail) (USD $)	3 Months Ended		9 Months Ended
	Sep. 30, 2011	Sep. 30, 2010	Sep. 30, 2011	Sep. 30, 2010
Revenues:
Electricity	$ 86,815,000	$ 83,357,000	$ 246,273,000	$ 218,269,000
Cost of revenues:
Electricity	57,941,000	61,530,000	186,090,000	179,551,000
Energy And Capacity
Revenues:
Electricity	33,313,000	30,113,000	93,194,000	80,460,000
Cost of revenues:
Electricity	28,840,000	31,752,000	96,732,000	95,710,000
Lease Portion Of Energy And Capacity
Revenues:
Electricity	52,831,000	52,573,000	151,065,000	135,795,000
Cost of revenues:
Electricity	27,790,000	28,467,000	85,426,000	79,909,000
Lease Income
Revenues:
Electricity	671,000	671,000	2,014,000	2,014,000
Cost of revenues:
Electricity	$ 1,311,000	$ 1,311,000	$ 3,932,000	$ 3,932,000