Property, Plant and Equipment (details) (USD $) In Thousands, unless otherwise specified	Sep. 30, 2013	Dec. 31, 2012	Sep. 30, 2012
Property, Plant and Equipment, Net, by Type [Abstract]
Property, Plant, and Equipment, Gross	$ 2,865,860	$ 2,786,008	$ 2,755,729
Accumulated Depreciation, Depletion and Amortization, Property, Plant, and Equipment	846,346	812,396	798,510
Total property, plant, and equipment, net	2,019,514	1,973,612	1,957,219
Utility Plant [Member]
Property, Plant and Equipment, Net, by Type [Abstract]
Property, Plant, and Equipment, Gross	2,482,034	2,435,886	2,399,600
Public Utilities, Property, Plant and Equipment, Construction Work in Progress	80,325	46,831	53,017
Accumulated Depreciation, Depletion and Amortization, Property, Plant, and Equipment	818,644	789,201	776,812
Total property, plant, and equipment, net	1,743,715	1,693,516	1,675,805
Non Utility Plant [Member]
Property, Plant and Equipment, Net, by Type [Abstract]
Property, Plant, and Equipment, Gross	296,022	296,781	296,486
Public Utilities, Property, Plant and Equipment, Construction Work in Progress	7,479	6,510	6,626
Accumulated Depreciation, Depletion and Amortization, Property, Plant, and Equipment	27,702	23,195	21,698
Total property, plant, and equipment, net	$ 275,799	$ 280,096	$ 281,414