Electric Plant and Depreciation - Schedule of Major Classes of Electric Plant (Details) - USD ($) $ in Thousands	Sep. 30, 2016	Dec. 31, 2015
Electric plant:
Transmission plant	$ 1,175,051	$ 1,080,050
Distribution plant	544,363	457,988
General plant	15,950	15,655
Total plant in service	1,735,364	1,553,693
CWIP	113,517	121,602
Total electric plant	1,848,881	1,675,295
Accumulated depreciation	(259,189)	(240,764)
Electric plant, net	$ 1,589,692	$ 1,434,531