Property, Plant and Equipment (details) (USD $) In Thousands, unless otherwise specified	Jun. 30, 2013	Dec. 31, 2012	Jun. 30, 2012
Property, Plant and Equipment, Net, by Type [Abstract]
Property, Plant, and Equipment, Gross	$ 2,833,083	$ 2,786,008	$ 2,720,037
Accumulated Depreciation, Depletion and Amortization, Property, Plant, and Equipment	833,851	812,396	791,021
Total property, plant, and equipment, net	1,999,232	1,973,612	1,929,016
Utility Plant [Member]
Property, Plant and Equipment, Net, by Type [Abstract]
Property, Plant, and Equipment, Gross	2,468,853	2,435,886	2,363,061
Public Utilities, Property, Plant and Equipment, Construction Work in Progress	61,283	46,831	54,039
Accumulated Depreciation, Depletion and Amortization, Property, Plant, and Equipment	807,652	789,201	770,825
Total property, plant, and equipment, net	1,722,484	1,693,516	1,646,275
Non Utility Plant [Member]
Property, Plant and Equipment, Net, by Type [Abstract]
Property, Plant, and Equipment, Gross	296,167	296,781	296,619
Public Utilities, Property, Plant and Equipment, Construction Work in Progress	6,780	6,510	6,318
Accumulated Depreciation, Depletion and Amortization, Property, Plant, and Equipment	26,199	23,195	20,196
Total property, plant, and equipment, net	$ 276,748	$ 280,096	$ 282,741