Derivatives and Fair Value Measurements - Amount of Gain or Losses Recognized in Income on Derivatives (Detail) - Cash Flow Hedging [Member] - Net Cost of Gas Sold [Member] - USD ($) $ in Thousands	3 Months Ended		12 Months Ended
	Mar. 31, 2018	Mar. 31, 2017	Mar. 31, 2018	Mar. 31, 2017
Derivative Instruments, Gain (Loss) [Line Items]
Amount of gain or (loss) recognized in income on derivative	$ (5,196)	$ (5,137)	$ (11,631)	$ 1,081
Regulatory Deferral Accounting Treatment [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Amount of gain or (loss) recognized in income on derivative	$ 5,196	$ 5,137	$ 11,631	$ (1,081)