Electric Plant and Depreciation - Additional Information (Details) (USD $) In Millions, unless otherwise specified	0 Months Ended
	Dec. 30, 2013 mi	Mar. 31, 2015	Dec. 31, 2014
Public Utility Property Plant And Equipment [Line Items]
Electric plant, net includes plant acquisition adjustments		$ 28.9	$ 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	2