Electric Plant and Depreciation - Schedule of Major Classes of Electric Plant (Details) - USD ($) $ in Thousands	Sep. 30, 2018	Dec. 31, 2017
Electric plant:
Transmission plant	$ 1,784,175	$ 1,685,466
Distribution plant	151,458	143,865
General plant	3,023	3,023
Total plant in service	1,938,656	1,832,354
Construction work in progress	60,027	113,643
Total electric plant	1,998,683	1,945,997
Accumulated depreciation	(198,166)	(173,768)
Electric plant, net	$ 1,800,517	$ 1,772,229