Regulatory Matters - Summary of Utilities Rate Plans (Details) - Rockland Electric Company (RECO) - USD ($) $ in Millions	1 Months Ended	6 Months Ended
	Jan. 31, 2016	Jun. 30, 2017
Public Utilities, General Disclosures [Line Items]
Base rate changes		$ 1.7
Amortization to income of net regulatory liabilities		$ 0.2
Regulatory liabilities, amortization period		3 years
Amortization to income of net regulatory assets		$ (25.6)
Regulatory assets, amortization period		4 years
Average rate base		$ 178.7
Weighted average cost of capital (after-tax) (percent)		7.47%
Authorized return on common equity (percent)		9.60%
Cost of long-term debt (percent)		5.37%
Common equity ratio (percent)		49.70%
Electric
Public Utilities, General Disclosures [Line Items]
Plan electric system storm hardening amount	$ 15.7
Plan period (years)	3 years