Deferred Assets and Other Regulatory Assets, Net - Additional Information (Details) - USD ($)	Sep. 30, 2015	Dec. 31, 2014	Dec. 10, 2014
Deferred Finance Costs And Other Regulatory Assets Net [Line Items]
Issuance of aggregate principal amount	$ 25,000,000
Deferred costs recoverable in future years	23,793,000	$ 23,793,000
Sharyland Distribution & Transmission Services
Deferred Finance Costs And Other Regulatory Assets Net [Line Items]
Credit facility, maximum borrowing capacity	$ 230,000,000
Revolving Credit Facility | Sharyland Distribution & Transmission Services
Deferred Finance Costs And Other Regulatory Assets Net [Line Items]
Credit facility, maximum borrowing capacity			$ 250,000,000
Senior Notes
Deferred Finance Costs And Other Regulatory Assets Net [Line Items]
Long-term debt, stated interest rate	8.50%