Regulatory Matters - Summary of Utilities Rate Plans (O&R New York-Electric) (Detail) - Electric - O&R - USD ($)	12 Months Ended				24 Months Ended	36 Months Ended
	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012	Oct. 31, 2017	Jun. 30, 2015
Public Utilities, General Disclosures [Line Items]
Deferred revenues		$ (3,400,000)	$ 3,200,000	$ 2,600,000
Negative revenue adjustments	$ 1,250,000.00	0	0	0
Deferral of net increase (decrease) to regulatory assets	1,200,000	(200,000)	4,100,000	7,800,000
Deferred regulatory liability (reduction)	2,200,000	(2,300,000)	$ (1,100,000)	$ 4,200,000
Earnings sharing, threshold limit	$ 0	$ 1,000,000
Maximum
Public Utilities, General Disclosures [Line Items]
Potential Penalties (annually) (up to)						$ 3,000,000
Scenario, Forecast
Public Utilities, General Disclosures [Line Items]
Deferred storm and property reserve deficiency, noncurrent					$ 59,300,000
Deferred storm and property reserve deficiency, recovery period (years)					5 years
Deferred storm and property reserve deficiency not recovered					$ 1,000,000
Scenario, Forecast | Maximum
Public Utilities, General Disclosures [Line Items]
Potential Penalties (annually) (up to)					4,000,000
Year 1
Public Utilities, General Disclosures [Line Items]
Base rate changes						19,400,000
Amortization to income of net regulatory (assets) and liabilities						(32,200,000)
Deferred regulatory liability (reduction)						678,000,000
Average rate base						$ 671,000,000
Weighted average cost of capital (after-tax) (percent)						7.61%
Authorized return on common equity (percent)						9.40%
Cost of long-term debt (percent)						6.07%
Common equity ratio (percent)						48.00%
Year 1 | Scenario, Forecast
Public Utilities, General Disclosures [Line Items]
Base rate changes					9,300,000
Amortization to income of net regulatory (assets) and liabilities					(8,500,000)
Deferred regulatory liability (reduction)					928,000,000
Average rate base					$ 763,000,000
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%
Deferred storm and property reserve deficiency, noncurrent					$ 11,850,000
Rate exclusion amount with balance below regulatory threshold					1,000,000
Year 2
Public Utilities, General Disclosures [Line Items]
Base rate changes						$ 8,800,000
Amortization to income of net regulatory (assets) and liabilities						(32,200,000)
Deferred regulatory liability (reduction)						704,000,000
Average rate base						$ 708,000,000
Weighted average cost of capital (after-tax) (percent)						7.65%
Authorized return on common equity (percent)						9.50%
Cost of long-term debt (percent)						6.07%
Common equity ratio (percent)						48.00%
Year 2 | Scenario, Forecast
Public Utilities, General Disclosures [Line Items]
Base rate changes					8,800,000
Amortization to income of net regulatory (assets) and liabilities					(9,400,000)
Deferred regulatory liability (reduction)					970,000,000
Average rate base					$ 805,000,000
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%
Deferred storm and property reserve deficiency, noncurrent					$ 11,850,000
Rate exclusion amount with balance below regulatory threshold					9,000,000
Year 3
Public Utilities, General Disclosures [Line Items]
Base rate changes						$ 15,200,000
Amortization to income of net regulatory (assets) and liabilities						(32,200,000)
Deferred regulatory liability (reduction)						753,000,000
Average rate base						$ 759,000,000
Weighted average cost of capital (after-tax) (percent)						7.48%
Authorized return on common equity (percent)						9.60%
Cost of long-term debt (percent)						5.64%
Common equity ratio (percent)						48.00%
Property Tax and Interest Rate Reconciliations | Scenario, Forecast
Public Utilities, General Disclosures [Line Items]
Regulatory assets not recoverable					$ 4,000,000