Summarized Financial Information Concerning the Company's Reportable Segments (Detail) (USD $) In Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2011	Jun. 30, 2010	Jun. 30, 2011	Jun. 30, 2010	Dec. 31, 2010
Segment Reporting Information [Line Items]
Net revenues from external customers	$ 104,614	$ 96,266	$ 202,434	$ 178,920
Intersegment revenues	17,387	21,102	30,749	28,296
Operating income	19,374	2,307	22,493	4,512
Segment assets at period end *	2,110,813	1,940,759	2,110,813	1,940,759	2,043,328
* Including unconsolidated investments	4,068	29,876	4,068	29,876
Electricity
Segment Reporting Information [Line Items]
Net revenues from external customers	81,190	68,807	159,458	134,912
Operating income	9,827	(5,109)	13,831	(2,014)
Segment assets at period end *	2,017,476	1,867,982	2,017,476	1,867,982
* Including unconsolidated investments	2,287	29,876	2,287	29,876
Product
Segment Reporting Information [Line Items]
Net revenues from external customers	23,424	27,459	42,976	44,008
Intersegment revenues	17,387	21,102	30,749	28,296
Operating income	9,547	7,416	8,662	6,526
Segment assets at period end *	93,337	72,777	93,337	72,777
* Including unconsolidated investments	$ 1,781		$ 1,781