Electric Plant and Depreciation - Additional Information (Details) $ in Millions	Dec. 30, 2013 Substations mi	Sep. 30, 2015 USD ($)	Dec. 31, 2014 USD ($)
Public Utility Property Plant And Equipment [Line Items]
Electric plant, net includes plant acquisition adjustments | $		$ 28.8	$ 28.7
Electric Plant Held For Future Use
Public Utility Property Plant And Equipment [Line Items]
Electric plant, transmission lines	66
Electric Plant Held For Future Use | Stanton
Public Utility Property Plant And Equipment [Line Items]
Number of substations | Substations	2