Electric Plant and Depreciation - Schedule of Major Classes of Electric Plant (Details) - USD ($) $ in Thousands	Jun. 30, 2016	Dec. 31, 2015
Electric plant:
Transmission plant	$ 1,134,030	$ 1,080,050
Distribution plant	519,953	457,988
General plant	15,951	15,655
Total plant in service	1,669,934	1,553,693
CWIP	122,612	121,602
Total electric plant	1,792,546	1,675,295
Accumulated depreciation	(252,824)	(240,764)
Electric plant, net	$ 1,539,722	$ 1,434,531