Regulatory Matters - Summary of Utilities Rate Plans (Rockland Electric Company (RECO)) (Detail) (Rockland Electric Company (RECO) [Member], USD $)	12 Months Ended				48 Months Ended	51 Months Ended	12 Months Ended	36 Months Ended	48 Months Ended
	Apr. 30, 2014	Apr. 30, 2013	Apr. 30, 2012	Apr. 30, 2011	Apr. 30, 2014	Jul. 31, 2014	Jul. 31, 2015	Jul. 31, 2015	Jul. 31, 2015
Public Utilities, General Disclosures [Line Items]
Base rate changes				$ 9.8
Amortizations to income of net regulatory (assets) and liabilities					(3.9)	(4.9)
Average rate base	148.6	148.6	148.6	148.6
Weighted average cost of capital (after-tax)	8.21%	8.21%	8.21%	8.21%
Authorized return on common equity	10.30%	10.30%	10.30%	10.30%
Cost of long-term debt	6.16%	6.16%	6.16%	6.16%	6.16%
Common equity ratio	50.00%	50.00%	50.00%	50.00%
Scenario, Forecast [Member]
Public Utilities, General Disclosures [Line Items]
Base rate changes							13.0
Amortizations to income of net regulatory (assets) and liabilities								0.4	(25.6)
Average rate base							$ 172.2
Weighted average cost of capital (after-tax)							7.83%
Authorized return on common equity							9.75%
Cost of long-term debt							5.89%	5.89%	5.89%
Common equity ratio							50.00%