Property, Plant and Equipment (Details) - USD ($) $ in Thousands	Jun. 30, 2017	Dec. 31, 2016	Jun. 30, 2016
Property, Plant and Equipment, Net, by Type [Abstract]
Property, plant and equipment, gross	$ 3,333,668	$ 3,208,816	$ 3,146,631
Less: Accumulated depreciation	973,084	947,916	932,179
Total property, plant, and equipment, net	2,360,584	2,260,900	2,214,452
Capital expenditures in accrued liabilities	42,684	9,547	11,345
Utility Plant
Property, Plant and Equipment, Net, by Type [Abstract]
Property, plant and equipment, gross	2,901,791	2,843,243	2,783,883
Work in progress	127,383	62,264	57,068
Less: Accumulated depreciation	925,589	903,096	890,028
Total property, plant, and equipment, net	2,103,585	2,002,411	1,950,923
Non Utility Plant
Property, Plant and Equipment, Net, by Type [Abstract]
Property, plant and equipment, gross	299,366	299,378	297,809
Work in progress	5,128	3,931	7,871
Less: Accumulated depreciation	47,495	44,820	42,151
Total property, plant, and equipment, net	$ 256,999	$ 258,489	$ 263,529