Electric Plant and Depreciation - Schedule of Major Classes of Electric Plant (Details) - USD ($) $ in Thousands	Mar. 31, 2017	Dec. 31, 2016
Electric plant:
Transmission plant	$ 1,222,769	$ 1,203,164
Distribution plant	597,707	575,648
General plant	15,958	15,959
Total plant in service	1,836,434	1,794,771
Construction work in progress	107,900	107,189
Total electric plant	1,944,334	1,901,960
Accumulated depreciation	(269,113)	(261,140)
Electric plant, net	$ 1,675,221	$ 1,640,820