SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES - Narrative (Details) $ / shares in Units, $ in Thousands, customer in Millions		3 Months Ended		9 Months Ended
	Jul. 19, 2019 USD ($)	Sep. 30, 2019 USD ($)	Sep. 30, 2018 USD ($)	Sep. 30, 2019 USD ($) utility customer $ / shares MW	Sep. 30, 2018 USD ($)	Feb. 08, 2019 USD ($) mi²	Jan. 01, 2019 USD ($)	Dec. 31, 2018 USD ($)
Summary of Accounting Policies [Line Items]
Number of electric, natural gas and water customers | customer				4,000,000
Operating lease liability		$ 52,300		$ 52,300
Operating lease right-of-use asset		52,300		52,300
Pre-tax impairment charge		0	$ 0	239,644	$ 0
After-tax impact of impairment charge				$ 204,400
After-tax impact of impairment charge (in dollars per share) | $ / shares				$ 0.64
Equity investment balance		861,687		$ 861,687				$ 464,286
Energy efficiency funds collected from customers to be remitted to the State of Connecticut			$ 10,700	21,400	36,100
Capital contribution				$ 374,913	$ 27,342
Northern Pass
Summary of Accounting Policies [Line Items]
Capacity required for transmission line | MW				1,090
Pre-tax impairment charge	$ 318,000
Offshore Wind Business
Summary of Accounting Policies [Line Items]
Ownership interest						50.00%
Equity investment balance		637,200		$ 637,200
Capital contribution		133,600
Revolution Wind and South Fork Wind
Summary of Accounting Policies [Line Items]
Cost of investment in unconsolidated affiliate						$ 225,000
Ownership interest						50.00%
Area of land (in square miles) | mi²						257
Bay State Wind
Summary of Accounting Policies [Line Items]
Ownership interest						50.00%
ASU 2016-02
Summary of Accounting Policies [Line Items]
Operating lease liability							$ 58,000
Operating lease right-of-use asset							58,000
NSTAR Electric
Summary of Accounting Policies [Line Items]
Operating lease liability		24,200		24,200
Operating lease right-of-use asset		24,200		24,200
NSTAR Electric | ASU 2016-02
Summary of Accounting Policies [Line Items]
Operating lease liability							25,300
Operating lease right-of-use asset							25,300
CL&P
Summary of Accounting Policies [Line Items]
Operating lease liability		900		900
Operating lease right-of-use asset		900		$ 900
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							600
Operating lease right-of-use asset							600
PSNH
Summary of Accounting Policies [Line Items]
Operating lease liability		400		$ 400
Operating lease right-of-use asset		$ 400		$ 400
PSNH | ASU 2016-02
Summary of Accounting Policies [Line Items]
Operating lease liability							600
Operating lease right-of-use asset							$ 600
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