Accounting for Derivative Instruments and Hedging Activities - Additional Information (Detail)	1 Months Ended		3 Months Ended
	Sep. 30, 2017	Nov. 30, 2016	Mar. 31, 2018 USD ($) MMBTU	Dec. 31, 2017 USD ($)	Dec. 31, 2016 USD ($)
Derivative [Line Items]
Derivative assets			$ 0
Derivative liabilities			0	$ 1,000,000	$ 1,000,000
Cash collateral posted with or received from any counterparties			0	$ 0
Net pre-tax gain (loss) expected to be reclassified from regulatory assets or liabilities, next 12 months			$ 0
Natural Gas [Member]
Derivative [Line Items]
Financial hedging moratorium period	5 years	1 year
Natural Gas Contracts [Member]
Derivative [Line Items]
Maximum length of time hedging in future cash flow			Nov. 30, 2018
Natural Gas Contracts [Member] | Financial [Member]
Derivative [Line Items]
Derivative volume | MMBTU			5