Property, Plant and Equipment (Details) - USD ($) $ in Thousands	Sep. 30, 2017	Dec. 31, 2016	Sep. 30, 2016
Property, Plant and Equipment, Net, by Type [Abstract]
Property, plant and equipment, gross	$ 3,384,122	$ 3,208,816	$ 3,177,196
Less: Accumulated depreciation	986,332	947,916	943,334
Total property, plant, and equipment, net	2,397,790	2,260,900	2,233,862
Capital expenditures in accrued liabilities	41,732	9,547	8,918
Utility Plant
Property, Plant and Equipment, Net, by Type [Abstract]
Property, plant and equipment, gross	2,934,424	2,843,243	2,815,340
Work in progress	145,148	62,264	58,470
Less: Accumulated depreciation	937,498	903,096	899,851
Total property, plant, and equipment, net	2,142,074	2,002,411	1,973,959
Non Utility Plant
Property, Plant and Equipment, Net, by Type [Abstract]
Property, plant and equipment, gross	300,224	299,378	298,586
Work in progress	4,326	3,931	4,800
Less: Accumulated depreciation	48,834	44,820	43,483
Total property, plant, and equipment, net	$ 255,716	$ 258,489	$ 259,903