Electric Plant and Depreciation - Additional Information (Details) $ in Thousands	3 Months Ended		12 Months Ended
	Mar. 31, 2016 USD ($)	Mar. 31, 2015 USD ($)	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	$ 28,400			$ 28,600
Payments to acquire electric plant	$ 58,495	$ 67,210
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