Schedule III - Electric Plant and Accumulated Depreciation - Components of Electric Plant and Accumulated Depreciation Fixed Asset Reconciliation (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015
Electric plant
Beginning balance	$ 1,675,295	$ 1,447,247
Additions	239,154	235,263
Retirements	(12,489)	(7,215)
Ending balance	1,901,960	1,675,295
Accumulated depreciation
Beginning balance	240,764	220,101
Depreciation expense	46,704	40,211
Retirements	(12,489)	(7,215)
Cost of removal	(13,839)	(12,333)
Ending balance	261,140	240,764
Electric plant, net	$ 1,640,820	$ 1,434,531