Regulatory Assets and Liabilities - Additional Information (Detail) - USD ($) $ in Millions	6 Months Ended
	Jun. 30, 2015	Jun. 27, 2014
Regulatory Assets and Liabilities [Line Items]
Amount Reclassified To Deferred Gas Cost		$ 2.5
Regulatory Asset, Amortization Period	amortized over a period of 2 to 3 years.
Property Taxes [Member]
Regulatory Assets and Liabilities [Line Items]
Regulatory Asset, Amortization Period	over a five year period beginning July 1, 2014