Accounting for Derivative Instruments and Hedging Activities - Additional Information (Detail) - USD ($)	1 Months Ended		3 Months Ended		9 Months Ended
	Sep. 30, 2017	Nov. 30, 2016	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016	Dec. 31, 2016
Derivative [Line Items]
Derivative assets	$ 0		$ 0		$ 0		$ 17,000,000
Derivative liabilities	0		0		0
Cash collateral posted with or received from any counterparties	0		0		0		$ 0
Net pre-tax gain (loss) expected to be reclassified from regulatory assets or liabilities, next 12 months					0
Gain on cash flow hedges			0	$ 1,000,000	1,000,000	$ 1,000,000
Natural Gas Contracts [Member]
Derivative [Line Items]
Derivative assets	0		0		0
Derivative liabilities	$ 0		0		$ 0
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	$ 1,000,000	$ 1,000,000
Natural Gas [Member]
Derivative [Line Items]
Financial hedging moratorium period	5 years	1 year