Property, Plant and Equipment (Details) - USD ($) $ in Thousands	Mar. 31, 2016	Dec. 31, 2015	Mar. 31, 2015
Property, Plant and Equipment, Net, by Type [Abstract]
Property, plant and equipment, gross	$ 3,115,854	$ 3,089,380	$ 3,017,754
Accumulated Depreciation, Depletion and Amortization, Property, Plant, and Equipment	919,187	906,717	883,254
Total property, plant, and equipment, net	2,196,667	2,182,663	2,134,500
Property, Plant, Equipment non-cash	8,424	8,985	8,451
Utility Plant [Member]
Property, Plant and Equipment, Net, by Type [Abstract]
Property, plant and equipment, gross	2,760,188	2,745,485	2,676,280
Public Utilities, Property, Plant and Equipment, Construction Work in Progress	51,014	39,288	34,048
Accumulated Depreciation, Depletion and Amortization, Property, Plant, and Equipment	878,364	867,377	847,278
Total property, plant, and equipment, net	1,932,838	1,917,396	1,863,050
Non Utility Plant [Member]
Property, Plant and Equipment, Net, by Type [Abstract]
Property, plant and equipment, gross	296,826	296,839	299,969
Public Utilities, Property, Plant and Equipment, Construction Work in Progress	7,826	7,768	7,457
Accumulated Depreciation, Depletion and Amortization, Property, Plant, and Equipment	40,823	39,340	35,976
Total property, plant, and equipment, net	$ 263,829	$ 265,267	$ 271,450