SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES - Narrative (Details) customer in Millions, $ in Millions	3 Months Ended
	Mar. 31, 2019 USD ($) customer utility MW	Mar. 31, 2018 USD ($)	Feb. 08, 2019 USD ($) mi²	Jan. 01, 2019 USD ($)
Summary of Accounting Policies [Line Items]
Number of electric, natural gas and water customers | customer	4
Operating lease liability	$ 56.7
Operating lease right-of-use assets	56.7
Energy efficiency funds collected from customers to be remitted to the State of Connecticut	$ 10.7	$ 12.7
Northern Pass
Summary of Accounting Policies [Line Items]
Capacity required for transmission line | MW	1,090
Captialized costs for the NPT project	$ 311.0
Revolution Wind and South Fork Wind
Summary of Accounting Policies [Line Items]
Cost of investment in unconsolidated affiliate			$ 225.0
Ownership interest			50.00%
Area of land | mi²			257
ASU 2016-02
Summary of Accounting Policies [Line Items]
Operating lease liability				$ 58.0
Operating lease right-of-use assets				58.0
NSTAR Electric
Summary of Accounting Policies [Line Items]
Operating lease liability	25.1
Operating lease right-of-use assets	25.1
NSTAR Electric | ASU 2016-02
Summary of Accounting Policies [Line Items]
Operating lease liability				25.3
Operating lease right-of-use assets				25.3
CL&P
Summary of Accounting Policies [Line Items]
Operating lease liability	0.5
Operating lease right-of-use assets	$ 0.5
Period of accounts receivable recoverable under financial or medical duress	180 days
CL&P | ASU 2016-02
Summary of Accounting Policies [Line Items]
Operating lease liability				0.6
Operating lease right-of-use assets				0.6
PSNH
Summary of Accounting Policies [Line Items]
Operating lease liability	$ 0.5
Operating lease right-of-use assets	$ 0.5
PSNH | ASU 2016-02
Summary of Accounting Policies [Line Items]
Operating lease liability				0.6
Operating lease right-of-use assets				$ 0.6
Yankee Gas
Summary of Accounting Policies [Line Items]
Period of accounts receivable recoverable under financial or medical duress	90 days
Connecticut, Massachusetts and New Hampshire
Summary of Accounting Policies [Line Items]
Number of regulated utilities | utility	8