Property, Plant and Equipment (details) (USD $) In Thousands, unless otherwise specified	Jun. 30, 2012	Dec. 31, 2011	Jun. 30, 2011
Property Plant And Equipment Net By Type Abstract
Property, plant and equipment	$ 2,720,037	$ 2,661,102	$ 2,612,147
Less: Accumulated depreciation	791,021	767,226	744,929
Total property, plant and equipment - net	1,929,016	1,893,876	1,867,218
Utility Plant [Member]
Property Plant And Equipment Net By Type Abstract
Property, plant and equipment	2,363,061	2,323,467	2,281,407
Public Utilities Property Plant And Equipment Construction Work In Progress	54,039	36,051	32,814
Less: Accumulated depreciation	770,825	749,603	730,199
Total property, plant and equipment - net	1,646,275	1,609,915	1,584,022
Non Utility Plant [Member]
Property Plant And Equipment Net By Type Abstract
Property, plant and equipment	296,619	293,205	290,035
Public Utilities Property Plant And Equipment Construction Work In Progress	6,318	8,379	7,891
Less: Accumulated depreciation	20,196	17,623	14,730
Total property, plant and equipment - net	$ 282,741	$ 283,961	$ 283,196