Electric Plant and Depreciation - Schedule of Major Classes of Electric Plant (Details) - USD ($) $ in Thousands	Mar. 31, 2019	Dec. 31, 2018
Electric plant:
Transmission plant	$ 1,794,442	$ 1,794,438
Distribution plant	151,698	151,698
General plant	3,210	3,023
Total plant in service	1,949,350	1,949,159
Construction work in progress	77,568	66,121
Total electric plant	2,026,918	2,015,280
Accumulated depreciation	(212,334)	(203,963)
Electric plant, net	$ 1,814,584	$ 1,811,317