SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES - Narrative (Details) $ in Thousands, customer in Millions	6 Months Ended
	Jun. 30, 2022 USD ($) utility customer	May 04, 2022	Mar. 31, 2022 USD ($)	Dec. 31, 2021 USD ($)	Jun. 30, 2021 USD ($)	Mar. 31, 2021 USD ($)	Dec. 31, 2020 USD ($)
Summary of Accounting Policies [Line Items]
Investments in unconsolidated affiliates	$ 1,719,878			$ 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	$ 457,800		$ 432,200	417,400	$ 425,800	$ 391,700	$ 358,900
Cash, cash equivalents, restricted cash and restricted cash equivalents	156,676			221,008	368,011		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	63,100			55,300
CL&P
Summary of Accounting Policies [Line Items]
Allowance for uncollectible accounts for late fees and other receivable amounts	$ 192,700		180,500	181,300	188,800	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	$ 17,911			74,788	207,441		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	100,500		94,800	97,000	98,000	89,300	91,600
Cash, cash equivalents, restricted cash and restricted cash equivalents	22,724			18,179	17,898		17,410
NSTAR Electric | COVID-19
Summary of Accounting Policies [Line Items]
Allowance for uncollectible accounts for late fees and other receivable amounts	7,500			9,000
PSNH
Summary of Accounting Policies [Line Items]
Allowance for uncollectible accounts for late fees and other receivable amounts	27,300		$ 26,200	24,300	17,200	$ 17,300	17,200
Cash, cash equivalents, restricted cash and restricted cash equivalents	34,299			35,126	$ 39,226		$ 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]
Ownership interest		50.00%
North East Offshore LLC
Summary of Accounting Policies [Line Items]
Ownership interest	50.00%