Regulatory Matters - Orange and Rockland Utilities, Inc. - Gas - Additional Information (Detail) (O & R [Member], USD $) In Millions, unless otherwise specified	12 Months Ended
	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
Schedule Of Regulatory Assets And Liabilities [Line Items]
Return on common equity	10.40%
Recognition of deferred expenses	$ 0.7	$ 2.9	$ 3.1
Deferred Revenue			1.5
Deferred expense	0.7	1.0
Annual inflation rate	4.00%
Accrued revenue	4.7	2.8	0.8
November 2009 Joint Proposal [Member]
Schedule Of Regulatory Assets And Liabilities [Line Items]
Increase rate	9.0
November Two Thousand Ten [Member]
Schedule Of Regulatory Assets And Liabilities [Line Items]
Increase rate	9.0
November Two Thousand Eleven [Member]
Schedule Of Regulatory Assets And Liabilities [Line Items]
Increase rate	4.6
Surcharge payment	4.3
Minimum [Member]
Schedule Of Regulatory Assets And Liabilities [Line Items]
Return on common equity	11.40%
Maximum [Member]
Schedule Of Regulatory Assets And Liabilities [Line Items]
Actual average common equity ratio	50.00%
Annual negative earnings adjustments	$ 1.4