Property, Plant and Equipment (details) (USD $) In Thousands	Jun. 30, 2011	Dec. 31, 2010	Jun. 30, 2010
Property Plant And Equipment Net By Type Abstract
Property, plant and equipment	$ 2,612,147	$ 2,576,402	$ 2,482,826
Less: Accumulated depreciation	744,929	722,239	710,732
Total property, plant and equipment - net	1,867,218	1,854,163	1,772,094
Utility Plant [Member]
Property Plant And Equipment Net By Type Abstract
Property, plant and equipment	2,281,407	2,247,952	2,218,660
Public Utilities Property Plant And Equipment Construction Work In Progress	32,814	29,324	30,086
Less: Accumulated depreciation	730,199	710,214	700,202
Total property, plant and equipment - net	1,584,022	1,567,062	1,548,544
Non Utility Plant [Member]
Property Plant And Equipment Net By Type Abstract
Property, plant and equipment	290,035	290,038	66,862
Public Utilities Property Plant And Equipment Construction Work In Progress	7,891	9,088	167,218
Less: Accumulated depreciation	14,730	12,025	10,530
Total property, plant and equipment - net	$ 283,196	$ 287,101	$ 223,550