Electric Plant and Depreciation - Schedule of Major Classes of Electric Plant (Details) - USD ($) $ in Thousands	Dec. 31, 2016	Dec. 31, 2015
Electric plant:
Transmission plant	$ 1,203,164	$ 1,080,050
Distribution plant	575,648	457,988
General plant	15,959	15,655
Total plant in service	1,794,771	1,553,693
CWIP	107,189	121,602
Total electric plant	1,901,960	1,675,295
Accumulated depreciation	(261,140)	(240,764)
Electric plant, net	$ 1,640,820	$ 1,434,531