Regulatory Matters - Summary of Utilities Rate Plans (O&R New York-Gas) (Detail) - USD ($)	12 Months Ended				36 Months Ended	62 Months Ended
	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012	Oct. 31, 2018	Dec. 31, 2014
Public Utilities, General Disclosures [Line Items]
Cost reconciliations, deferred net regulatory assets	$ 836,000,000	$ 615,000,000				$ 615,000,000
Gas | O&R
Public Utilities, General Disclosures [Line Items]
Deferred revenues	(800,000)	(100,000)	$ 700,000	$ 4,700,000		(100,000)
Negative revenue adjustments	0	0	0	0
Cost reconciliations, deferred net regulatory assets	2,500,000	8,300,000	8,300,000	700,000		8,300,000
Net utility plant reconciliations	0	0	0	700,000
Earnings sharing, threshold limit	$ 0	$ 0	$ 0	$ 0
Gas | O&R | Maximum
Public Utilities, General Disclosures [Line Items]
Potential Penalties (annually) (up to)						1,400,000.0
Scenario, Forecast | Property Tax and Interest Rate Reconciliations | Gas | O&R
Public Utilities, General Disclosures [Line Items]
Regulatory assets not recoverable					$ 14,000,000
Year 1 | Gas | O&R
Public Utilities, General Disclosures [Line Items]
Base rate changes						9,000,000
Amortization to income of net regulatory (assets) and liabilities						(2,000,000)
Average rate base						$ 280,000,000
Weighted average cost of capital (after-tax) (percent)						8.49%
Authorized return on common equity (percent)						10.40%
Earnings sharing (percentage)		11.40%				11.40%
Cost of long-term debt (percent)		6.81%				6.81%
Common equity ratio (percent)						48.00%
Year 1 | Scenario, Forecast | Gas | O&R
Public Utilities, General Disclosures [Line Items]
Base rate changes					16,400,000
Amortization to income of net regulatory (assets) and liabilities					(1,700,000)
Average rate base					$ 366,000,000
Weighted average cost of capital (after-tax) (percent)					7.10%
Authorized return on common equity (percent)					9.00%
Earnings sharing (percentage)					9.60%
Cost of long-term debt (percent)					5.42%
Common equity ratio (percent)					48.00%
Rate exclusion amount with balance below regulatory threshold					$ 500,000
Year 1 | Scenario, Forecast | Gas | O&R | Maximum
Public Utilities, General Disclosures [Line Items]
Potential Penalties (annually) (up to)					3,700,000.0
Net utility plant reconciliations					492,000,000
Year 2 | Gas | O&R
Public Utilities, General Disclosures [Line Items]
Base rate changes						$ 9,000,000
Amortization to income of net regulatory (assets) and liabilities						(2,000,000)
Average rate base						$ 296,000,000
Weighted average cost of capital (after-tax) (percent)						8.49%
Authorized return on common equity (percent)						10.40%
Earnings sharing (percentage)		11.40%				11.40%
Cost of long-term debt (percent)		6.81%				6.81%
Common equity ratio (percent)						48.00%
Year 2 | Scenario, Forecast | Gas | O&R
Public Utilities, General Disclosures [Line Items]
Base rate changes					16,400,000
Amortization to income of net regulatory (assets) and liabilities					(2,100,000)
Average rate base					$ 391,000,000
Weighted average cost of capital (after-tax) (percent)					7.06%
Authorized return on common equity (percent)					9.00%
Earnings sharing (percentage)					9.60%
Cost of long-term debt (percent)					5.35%
Common equity ratio (percent)					48.00%
Rate exclusion amount with balance below regulatory threshold					$ 4,200,000
Year 2 | Scenario, Forecast | Gas | O&R | Maximum
Public Utilities, General Disclosures [Line Items]
Potential Penalties (annually) (up to)					4,700,000.0
Net utility plant reconciliations					518,000,000
Year 3 | Gas | O&R
Public Utilities, General Disclosures [Line Items]
Base rate changes						$ 4,600,000
Base rate change through surcharge						4,300,000
Amortization to income of net regulatory (assets) and liabilities						(2,000,000)
Average rate base						$ 309,000,000
Weighted average cost of capital (after-tax) (percent)						8.49%
Authorized return on common equity (percent)						10.40%
Earnings sharing (percentage)		11.40%				11.40%
Cost of long-term debt (percent)		6.81%				6.81%
Common equity ratio (percent)						48.00%
Year 3 | Scenario, Forecast | Gas | O&R
Public Utilities, General Disclosures [Line Items]
Base rate changes					5,800,000
Base rate change through surcharge					10,600,000
Amortization to income of net regulatory (assets) and liabilities					(2,500,000)
Average rate base					$ 417,000,000
Weighted average cost of capital (after-tax) (percent)					7.06%
Authorized return on common equity (percent)					9.00%
Earnings sharing (percentage)					9.60%
Cost of long-term debt (percent)					5.35%
Common equity ratio (percent)					48.00%
Rate exclusion amount with balance below regulatory threshold					$ 7,200,000
Year 3 | Scenario, Forecast | Gas | O&R | Maximum
Public Utilities, General Disclosures [Line Items]
Potential Penalties (annually) (up to)					5,800,000.0
Net utility plant reconciliations					$ 546,000,000