SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES - Narrative (Details) customer in Millions	9 Months Ended
	Sep. 30, 2022 USD ($) regulatedUtility customer project	Jun. 30, 2022 USD ($)	May 04, 2022	Mar. 31, 2022 USD ($)	Dec. 31, 2021 USD ($)	Sep. 30, 2021 USD ($)	Jun. 30, 2021 USD ($)	Dec. 31, 2020 USD ($)
Summary of Accounting Policies [Line Items]
Investments in unconsolidated affiliates	$ 2,053,404,000				$ 1,436,293,000
Number of electric, natural gas and water customers | customer	4.4
Allowance for uncollectible accounts for late fees and other receivable amounts	$ 457,600,000	$ 457,800,000			417,400,000	$ 441,700,000	$ 425,800,000	$ 358,900,000
Number of offshore wind projects | project	3
Cash, cash equivalents, restricted cash and restricted cash equivalents	$ 602,256,000				$ 221,008,000	218,129,000		264,950,000
Restricted Cash, Noncurrent, Statement of Financial Position [Extensible Enumeration]	Other Long-Term Assets				Other Long-Term Assets
Energy Relief Fund - Merrimack Valley Programs
Summary of Accounting Policies [Line Items]
Restricted cash, current	$ 20,000,000
Restricted cash, noncurrent	15,900,000
Restricted cash					$ 41,500,000
COVID-19
Summary of Accounting Policies [Line Items]
Allowance for uncollectible accounts for late fees and other receivable amounts	55,500,000				55,300,000
CL&P
Summary of Accounting Policies [Line Items]
Allowance for uncollectible accounts for late fees and other receivable amounts	$ 198,600,000	192,700,000			181,300,000	195,500,000	188,800,000	157,400,000
Period of accounts receivable recoverable under financial or medical duress	180 days
Cash, cash equivalents, restricted cash and restricted cash equivalents	$ 23,120,000				74,788,000	71,762,000		99,809,000
CL&P | Customer Assistance Fund
Summary of Accounting Policies [Line Items]
Cash, cash equivalents, restricted cash and restricted cash equivalents				$ 10,000,000	10,000,000
CL&P | COVID-19
Summary of Accounting Policies [Line Items]
Allowance for uncollectible accounts for late fees and other receivable amounts	19,700,000				23,900,000
NSTAR Electric
Summary of Accounting Policies [Line Items]
Allowance for uncollectible accounts for late fees and other receivable amounts	99,500,000	100,500,000			97,000,000.0	105,000,000.0	98,000,000.0	91,600,000
Cash, cash equivalents, restricted cash and restricted cash equivalents	470,945,000				18,179,000	20,838,000		17,410,000
NSTAR Electric | COVID-19
Summary of Accounting Policies [Line Items]
Allowance for uncollectible accounts for late fees and other receivable amounts	4,100,000				9,000,000
PSNH
Summary of Accounting Policies [Line Items]
Allowance for uncollectible accounts for late fees and other receivable amounts	28,800,000	$ 27,300,000			24,300,000	17,000,000.0	$ 17,200,000	17,200,000
Cash, cash equivalents, restricted cash and restricted cash equivalents	24,246,000				35,126,000	$ 26,173,000		$ 39,555,000
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 | regulatedUtility	12
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%