Regulatory Assets and Liabilities - Additional Information (Detail) (USD $) In Millions, unless otherwise specified	12 Months Ended
	Dec. 31, 2014	Jun. 27, 2014
Regulatory Assets and Liabilities [Line Items]
Amount Reclassified To Deferred Gas Cost		$ 2.5
Regulatory Noncurrent Asset, Amortization Period	amortized over a period of 2 to 3 years
Property Taxes [Member]
Regulatory Assets and Liabilities [Line Items]
Regulatory Noncurrent Asset, Amortization Period	over a ten-year period starting January 1, 2004