Accounting for Derivative Instruments and Hedging Activities - Additional Information (Detail) - USD ($)	3 Months Ended
	Mar. 31, 2017	Mar. 31, 2016	Dec. 31, 2016
Derivative [Line Items]
Derivative assets	$ 6,000,000		$ 17,000,000
Derivative liabilities	0		0
Cash collateral posted with or received from any counterparties	0		$ 0
Net pretax gain (loss) expected to be reclassified from regulatory assets or liabilities, next 12 months	$ 6,000,000
Natural Gas Contracts [Member]
Derivative [Line Items]
Maximum length of time hedging in future cash flow	Nov. 30, 2018
Maximum [Member]
Derivative [Line Items]
Gain on cash flow hedges	$ 1,000,000	$ 1,000,000