Electric Plant and Depreciation - Schedule of Major Classes of Electric Plant (Details) - USD ($) $ in Thousands	Sep. 30, 2015	Dec. 31, 2014
Electric plant:
Transmission plant	$ 1,073,862	$ 966,560
Distribution plant	439,098	387,329
General plant	15,617	15,018
Total plant in service	1,528,577	1,368,907
CWIP	85,649	41,222
Total electric plant	1,614,226	1,410,129
Accumulated depreciation	(244,726)	(220,101)
Electric plant held for future use		37,118
Electric plant, net	$ 1,369,500	$ 1,227,146