Derivatives and Fair Value Measurements - Amount of Gain or Losses Recognized in Income on Derivatives (Detail) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended		12 Months Ended
	Sep. 30, 2015	Sep. 30, 2014	Sep. 30, 2015	Sep. 30, 2014	Sep. 30, 2015	Sep. 30, 2014
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	(3,282)	(2,277)	(4,689)	3,630	(10,682)	7,552
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	$ 3,282	$ 2,277	$ 4,689	$ (3,630)	$ 10,682	$ (7,552)