Electric Plant and Depreciation - Schedule of Major Classes of Electric Plant (Details) - USD ($) $ in Thousands	Jun. 30, 2017	Dec. 31, 2016
Electric plant:
Transmission plant	$ 1,236,848	$ 1,203,164
Distribution plant	614,174	575,648
General plant	15,958	15,959
Total plant in service	1,866,980	1,794,771
Construction work in progress	111,706	107,189
Total electric plant	1,978,686	1,901,960
Accumulated depreciation	(268,266)	(261,140)
Electric plant, net	$ 1,710,420	$ 1,640,820