Electric Plant and Depreciation - Schedule of Major Classes of Electric Plant (Details) - USD ($) $ in Thousands	Mar. 31, 2016	Dec. 31, 2015
Electric plant:
Transmission plant	$ 1,133,382	$ 1,080,050
Distribution plant	476,406	457,988
General plant	15,951	15,655
Total plant in service	1,625,739	1,553,693
CWIP	102,644	121,602
Total electric plant	1,728,383	1,675,295
Accumulated depreciation	(245,875)	(240,764)
Electric plant, net	$ 1,482,508	$ 1,434,531