Property, Plant and Equipment (details) (USD $) In Thousands	Sep. 30, 2011	Dec. 31, 2010	Sep. 30, 2010
Property Plant And Equipment Net By Type Abstract
Property, plant and equipment	$ 2,632,498	$ 2,576,402	$ 2,528,703
Less: Accumulated depreciation	756,592	722,239	711,046
Total property, plant and equipment - net	1,875,906	1,854,163	1,817,657
Utility Plant [Member]
Property Plant And Equipment Net By Type Abstract
Property, plant and equipment	2,296,788	2,247,952	2,222,222
Public Utilities Property Plant And Equipment Construction Work In Progress	36,459	29,324	33,359
Less: Accumulated depreciation	740,378	710,214	700,193
Total property, plant and equipment - net	1,592,869	1,567,062	1,555,388
Non Utility Plant [Member]
Property Plant And Equipment Net By Type Abstract
Property, plant and equipment	290,075	290,038	66,299
Public Utilities Property Plant And Equipment Construction Work In Progress	9,176	9,088	206,823
Less: Accumulated depreciation	16,214	12,025	10,853
Total property, plant and equipment - net	$ 283,037	$ 287,101	$ 262,269