Derivatives - Amount of Gain or Losses Recognized in Income on Derivatives (Detail) - Cash Flow Hedging - USD ($) $ in Thousands	3 Months Ended		12 Months Ended
	Mar. 31, 2019	Mar. 31, 2018	Mar. 31, 2019	Mar. 31, 2018
Derivative Instruments, Gain (Loss) [Line Items]
Amount of gain or (loss) recognized in income on derivative	$ 0	$ 0	$ 0	$ 0
Net cost of gas sold
Derivative Instruments, Gain (Loss) [Line Items]
Amount of gain or (loss) recognized in income on derivative	(3,533)	(5,196)	(450)	(11,631)
Regulatory Deferral Accounting Treatment | Net cost of gas sold
Derivative Instruments, Gain (Loss) [Line Items]
Amount of gain or (loss) recognized in income on derivative	$ 3,533	$ 5,196	$ 450	$ 11,631