Regulatory Matters - Summary of Utilities Rate Plans (O&R New York-Gas) (Detail) (USD $)	12 Months Ended						36 Months Ended
	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012	Oct. 31, 2012	Oct. 31, 2011	Oct. 31, 2010	Oct. 31, 2012
Public Utilities, General Disclosures [Line Items]
Cost reconciliations, deferred net regulatory assets	$ 615,000,000	$ 604,000,000
O&R [Member] | Gas [Member]
Public Utilities, General Disclosures [Line Items]
Base rate changes				4,600,000	9,000,000	9,000,000
Base rate change through surcharge				4,300,000
Amortizations to income of net regulatory (assets) and liabilities							(2,000,000)
Deferred revenues	(100,000)	700,000	4,700,000
Negative revenue adjustments	0	0	0
Cost reconciliations, deferred net regulatory assets	8,300,000	8,300,000	700,000
Net utility plant reconciliations	0	0	700,000
Average rate base				309,000,000	296,000,000	280,000,000
Weighted average cost of capital (after-tax)				8.49%	8.49%	8.49%
Authorized return on common equity				10.40%	10.40%	10.40%
Earnings sharing percentage				11.40%	11.40%	11.40%	11.40%
Cost of long-term debt				6.81%	6.81%	6.81%	6.81%
Common equity ratio				48.00%	48.00%	48.00%
O&R [Member] | Gas [Member] | Maximum [Member]
Public Utilities, General Disclosures [Line Items]
Potential penalties				$ 1,400,000	$ 1,400,000	$ 1,400,000