Derivatives and Fair Value Measurements - Amount of Gain or Losses Recognized in Income on Derivatives (Detail) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended		12 Months Ended
	Jun. 30, 2016	Jun. 30, 2015	Jun. 30, 2016	Jun. 30, 2015	Jun. 30, 2016	Jun. 30, 2015
Derivative Instruments, Gain (Loss) [Line Items]
Amount of gain or (loss) recognized in income on derivative	$ 0	$ 0	$ 0	$ 0	$ 0	$ 0
Cash Flow Hedging [Member] | Net Cost of Gas Sold [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Amount of gain or (loss) recognized in income on derivative	5,537	707	4,325	(1,407)	(1,866)	(9,677)
Regulatory Deferral Accounting Treatment [Member] | Cash Flow Hedging [Member] | Net Cost of Gas Sold [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Amount of gain or (loss) recognized in income on derivative	$ (5,537)	$ (707)	$ (4,325)	$ 1,407	$ 1,866	$ 9,677