Electric Plant and Depreciation - Additional Information (Details) $ in Thousands	9 Months Ended		12 Months Ended
	Sep. 30, 2016 USD ($)	Sep. 30, 2015 USD ($)	Dec. 31, 2013 USD ($) Substations mi	Dec. 31, 2015 USD ($)
Public Utility Property Plant And Equipment [Line Items]
Electric plant, net includes plant acquisition adjustments	$ 27,900			$ 28,600
Payments to acquire electric plant	$ 179,765	$ 164,890
Electric Plant Held For Future Use
Public Utility Property Plant And Equipment [Line Items]
Electric plant, transmission lines | mi			66
Electric Plant Held For Future Use | Stanton
Public Utility Property Plant And Equipment [Line Items]
Number of substations | Substations			2
Payments to acquire electric plant			$ 37,100