Summarized Financial Information Concerning the Company's Reportable Segments (Detail) (USD $) In Thousands	3 Months Ended				9 Months Ended
	Sep. 30, 2011		Sep. 30, 2010		Sep. 30, 2011		Sep. 30, 2010		Dec. 31, 2010
Segment Reporting Information [Line Items]
Net revenues from external customers	$ 110,841		$ 101,477		$ 313,275		$ 280,397
Intersegment revenues	15,264		10,977		46,013		39,273
Operating income	24,208		14,818		46,701		19,330
Segment assets at period end	2,210,721	[1]	1,964,709	[2]	2,210,721	[1]	1,964,709	[2]	2,043,328
Electricity
Segment Reporting Information [Line Items]
Net revenues from external customers	86,815		83,357		246,273		218,269
Operating income	21,087		13,461		34,917		11,447
Segment assets at period end	2,121,932	[1]	1,895,469	[2]	2,121,932	[1]	1,895,469	[2]
Product
Segment Reporting Information [Line Items]
Net revenues from external customers	24,026		18,120		67,002		62,128
Intersegment revenues	15,264		10,977		46,013		39,273
Operating income	3,121		1,357		11,784		7,883
Segment assets at period end	$ 88,789	[1]	$ 69,240	[2]	$ 88,789	[1]	$ 69,240	[2]

[1] Including unconsolidated investments 2,287 1,710 3,997

[2] Including unconsolidated investments 2,040 - 2,040