SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES - Narrative (Details) $ in Thousands, customer in Millions	3 Months Ended
	Mar. 31, 2022 USD ($) customer utility	May 04, 2022	Dec. 31, 2021 USD ($)	Mar. 31, 2021 USD ($)	Dec. 31, 2020 USD ($)
Summary of Accounting Policies [Line Items]
Investments in unconsolidated affiliates	$ 1,545,433		$ 1,436,293
Number of electric, natural gas and water customers | customer	4.4
Allowance for uncollectible accounts for late fees and other receivable amounts	$ 432,200		417,400	$ 391,700	$ 358,900
Cash, cash equivalents, restricted cash and restricted cash equivalents	164,589		221,008	175,604	264,950
Energy Relief Fund - Merrimack Valley Programs
Summary of Accounting Policies [Line Items]
Restricted cash, current	20,000
Energy Relief Fund - Merrimack Valley Programs | Other Long-Term Assets
Summary of Accounting Policies [Line Items]
Restricted cash, noncurrent	15,900
Restricted cash			41,500
COVID-19
Summary of Accounting Policies [Line Items]
Allowance for uncollectible accounts for late fees and other receivable amounts	58,500		55,300
CL&P
Summary of Accounting Policies [Line Items]
Allowance for uncollectible accounts for late fees and other receivable amounts	$ 180,500		181,300	175,100	157,400
Period of accounts receivable recoverable under financial or medical duress	180 days
Cash, cash equivalents, restricted cash and restricted cash equivalents	$ 30,584		74,788	18,288	99,809
CL&P | Customer Assistance Fund
Summary of Accounting Policies [Line Items]
Cash, cash equivalents, restricted cash and restricted cash equivalents	10,000		10,000
CL&P | COVID-19
Summary of Accounting Policies [Line Items]
Allowance for uncollectible accounts for late fees and other receivable amounts	20,400		23,900
NSTAR Electric
Summary of Accounting Policies [Line Items]
Allowance for uncollectible accounts for late fees and other receivable amounts	94,800		97,000	89,300	91,600
Cash, cash equivalents, restricted cash and restricted cash equivalents	18,144		18,179	17,373	17,410
NSTAR Electric | COVID-19
Summary of Accounting Policies [Line Items]
Allowance for uncollectible accounts for late fees and other receivable amounts	9,000		9,000
PSNH
Summary of Accounting Policies [Line Items]
Allowance for uncollectible accounts for late fees and other receivable amounts	26,200		24,300	17,300	17,200
Cash, cash equivalents, restricted cash and restricted cash equivalents	24,058		35,126	$ 27,988	$ 39,555
PSNH | COVID-19
Summary of Accounting Policies [Line Items]
Allowance for uncollectible accounts for late fees and other receivable amounts	$ 0		0
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	10
Offshore Wind Business
Summary of Accounting Policies [Line Items]
Investments in unconsolidated affiliates	$ 1,330,000		$ 1,210,000
Offshore Wind Business | Subsequent Event
Summary of Accounting Policies [Line Items]
Ownership interest		50.00%
North East Offshore LLC
Summary of Accounting Policies [Line Items]
Ownership interest	50.00%
North East Offshore LLC | Subsequent Event
Summary of Accounting Policies [Line Items]
Ownership interest		50.00%