Property, Plant and Equipment (details) (USD $) In Thousands, unless otherwise specified	Mar. 31, 2012	Dec. 31, 2011	Mar. 31, 2011
Property Plant And Equipment Net By Type Abstract
Property, plant and equipment	$ 2,680,537	$ 2,661,102	$ 2,593,553
Less: Accumulated depreciation	779,683	767,226	733,639
Total property, plant and equipment - net	1,900,854	1,893,876	1,859,914
Utility Plant [Member]
Property Plant And Equipment Net By Type Abstract
Property, plant and equipment	2,342,681	2,323,467	2,264,055
Public Utilities Property Plant And Equipment Construction Work In Progress	34,903	36,051	28,464
Less: Accumulated depreciation	760,566	749,603	720,134
Total property, plant and equipment - net	1,617,018	1,609,915	1,572,385
Non Utility Plant [Member]
Property Plant And Equipment Net By Type Abstract
Property, plant and equipment	297,164	293,205	292,089
Public Utilities Property Plant And Equipment Construction Work In Progress	5,789	8,379	8,945
Less: Accumulated depreciation	19,117	17,623	13,505
Total property, plant and equipment - net	$ 283,836	$ 283,961	$ 287,529