Property, Plant and Equipment (details) (USD $) In Thousands, unless otherwise specified	Mar. 31, 2013	Dec. 31, 2012	Mar. 31, 2012
Property, Plant and Equipment, Net, by Type [Abstract]
Property, Plant, and Equipment, Gross	$ 2,808,673	$ 2,786,008	$ 2,680,537
Accumulated Depreciation, Depletion and Amortization, Property, Plant, and Equipment	824,561	812,396	779,683
Total property, plant, and equipment, net	1,984,112	1,973,612	1,900,854
Utility Plant [Member]
Property, Plant and Equipment, Net, by Type [Abstract]
Property, Plant, and Equipment, Gross	2,452,419	2,435,886	2,342,681
Public Utilities, Property, Plant and Equipment, Construction Work in Progress	53,474	46,831	34,903
Accumulated Depreciation, Depletion and Amortization, Property, Plant, and Equipment	799,864	789,201	760,566
Total property, plant, and equipment, net	1,706,029	1,693,516	1,617,018
Non Utility Plant [Member]
Property, Plant and Equipment, Net, by Type [Abstract]
Property, Plant, and Equipment, Gross	296,228	296,781	297,164
Public Utilities, Property, Plant and Equipment, Construction Work in Progress	6,552	6,510	5,789
Accumulated Depreciation, Depletion and Amortization, Property, Plant, and Equipment	24,697	23,195	19,117
Total property, plant, and equipment, net	$ 278,083	$ 280,096	$ 283,836