Regulatory Matters - Summary of Utilities Rate Plans (O&R New York-Electric) (Detail) (O&R [Member], Electric Transmission [Member], USD $)	12 Months Ended					0 Months Ended	12 Months Ended	36 Months Ended
	Dec. 31, 2014	Jun. 30, 2014	Dec. 31, 2013	Jun. 30, 2013	Dec. 31, 2012	Nov. 01, 2015	Jun. 30, 2015	Jun. 30, 2015
Public Utilities, General Disclosures [Line Items]
Base rate changes		$ 8,800,000		$ 19,400,000
Deferred revenues	(3,400,000)		3,200,000		2,600,000
Negative revenue adjustments	0		0		0
Recognition of deferred expenses	(200,000)		4,100,000		7,800,000
Deferred regulatory liability	(2,300,000)	704,000,000	(1,100,000)	678,000,000	4,200,000
Average rate base		708,000,000		671,000,000
Weighted average cost of capital (after-tax)		7.65%		7.61%
Authorized return on common equity		9.50%		9.40%
Earnings sharing, threshold limit	1,000,000
Cost of long-term debt		6.07%		6.07%
Common equity ratio		48.00%		48.00%
Maximum [Member]
Public Utilities, General Disclosures [Line Items]
Potential penalties		3,000,000		3,000,000
Scenario, Forecast [Member]
Public Utilities, General Disclosures [Line Items]
Base rate changes							15,200,000
Amortizations to income of net regulatory (assets) and liabilities								(32,200,000)
Deferred regulatory liability							753,000,000
Average rate base							759,000,000
Weighted average cost of capital (after-tax)							7.48%
Authorized return on common equity						9.75%	9.60%
Cost of long-term debt							5.64%	5.64%
Common equity ratio						48.00%	48.00%
Scenario, Forecast [Member] | Maximum [Member]
Public Utilities, General Disclosures [Line Items]
Potential penalties							$ 3,000,000