Regulatory Matters - Summary of Utilities Rate Plans (Rockland Electric Company (RECO)) (Details) - Rockland Electric Company (RECO) - USD ($) $ in Millions	1 Months Ended	12 Months Ended	51 Months Ended
	Jan. 31, 2016	Jul. 31, 2015	Jul. 31, 2014
Public Utilities, General Disclosures [Line Items]
Average rate base		$ 172.2	$ 148.6
Weighted average cost of capital (after-tax) (percent)		7.83%	8.21%
Authorized return on common equity (percent)		9.75%	10.30%
Cost of long-term debt (percent)		5.89%	6.16%
Common equity ratio (percent)		50.00%	50.00%
Electric
Public Utilities, General Disclosures [Line Items]
RECO plan period (years)	3 years
RECO plan electric system storm hardening amount	$ 15.7
Year 1
Public Utilities, General Disclosures [Line Items]
Base rate changes		$ 13.0	$ 9.8
Amortization to income of net regulatory (assets) and liabilities		0.4	(3.9)
Deferred major storm costs		(25.6)	(4.9)
Year 2
Public Utilities, General Disclosures [Line Items]
Amortization to income of net regulatory (assets) and liabilities		0.4	(3.9)
Deferred major storm costs		(25.6)	(4.9)
Year 3
Public Utilities, General Disclosures [Line Items]
Amortization to income of net regulatory (assets) and liabilities		0.4	(3.9)
Deferred major storm costs		(25.6)	(4.9)
Year 4
Public Utilities, General Disclosures [Line Items]
Amortization to income of net regulatory (assets) and liabilities			(3.9)
Deferred major storm costs		$ (25.6)	(4.9)
Year 5
Public Utilities, General Disclosures [Line Items]
Deferred major storm costs			$ (4.9)