SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES - Narrative (Details) $ / shares in Units, $ in Thousands, customer in Millions		1 Months Ended	3 Months Ended		6 Months Ended
	Jul. 19, 2019 USD ($)	Jul. 31, 2019 USD ($)	Jun. 30, 2019 USD ($) $ / shares	Jun. 30, 2018 USD ($)	Jun. 30, 2019 USD ($) utility customer $ / shares MW	Jun. 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			$ 54,600		$ 54,600
Operating lease right-of-use asset			54,600		54,600
Pre-tax impairment charge			239,644	$ 0	239,644	$ 0
After-tax impact of impairment charge			$ 204,400		$ 204,400
After-tax impact of impairment charge (in dollars per share) | $ / shares			$ 0.64		$ 0.64
Equity investment balance			$ 748,705		$ 748,705				$ 464,286
Energy efficiency funds collected from customers to be remitted to the State of Connecticut			10,700	$ 12,700	21,400	25,400
Capital contribution					$ 265,955	$ 13,220
Northern Pass
Summary of Accounting Policies [Line Items]
Capacity required for transmission line | MW					1,090
Northern Pass | Subsequent Event
Summary of Accounting Policies [Line Items]
Pre-tax impairment charge	$ 318,000
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 | mi²							257
Equity investment balance			499,500		$ 499,500
Revolution Wind and South Fork Wind | Subsequent Event
Summary of Accounting Policies [Line Items]
Capital contribution		$ 54,900
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			24,400		24,400
Operating lease right-of-use asset			24,400		24,400
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			600		600
Operating lease right-of-use asset			600		$ 600
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			800		$ 800
Operating lease right-of-use asset			$ 800		$ 800
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