Components of Electricity Revenues and Cost of Revenues (Detail) (USD $)	3 Months Ended		6 Months Ended
	Jun. 30, 2011	Jun. 30, 2010	Jun. 30, 2011	Jun. 30, 2010
Revenues:
Electricity	$ 81,190,000	$ 68,807,000	$ 159,458,000	$ 134,912,000
Cost of revenues:
Electricity	62,212,000	63,498,000	128,149,000	118,021,000
Energy And Capacity
Revenues:
Electricity	28,726,000	25,628,000	59,881,000	50,346,000
Cost of revenues:
Electricity	32,371,000	36,702,000	67,892,000	63,957,000
Lease Portion Of Energy And Capacity
Revenues:
Electricity	51,792,000	42,507,000	98,234,000	83,223,000
Cost of revenues:
Electricity	28,531,000	25,486,000	57,636,000	51,443,000
Lease Income
Revenues:
Electricity	672,000	672,000	1,343,000	1,343,000
Cost of revenues:
Electricity	$ 1,310,000	$ 1,310,000	$ 2,621,000	$ 2,621,000