SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES - Narrative (Details) $ / shares in Units, $ in Thousands, customer in Millions		12 Months Ended
	Oct. 09, 2020 USD ($)	Dec. 31, 2021 USD ($) customer utility MW	Dec. 31, 2020 USD ($) MW	Dec. 31, 2019 USD ($) $ / shares
Summary of Accounting Policies [Line Items]
Number of electric and natural gas customers | customer		4.4
Goodwill		$ 4,477,269	$ 4,445,988	$ 4,427,000
Amount of power to be purchased under capacity-related contract (up to) (in MW) | MW			675
Pre-tax impairment charge		0	$ 0	239,644
Allowance for uncollectible accounts for late fees and other receivable amounts		417,406	358,851	224,800
Charitable contributions			6,400
Increase to operating cash inflows in accounts payable		29,201	(122,567)	(14,866)
Increase to investing cash outflows in investments in property, plant and equipment		3,175,080	2,942,996	2,911,489
Electric Transmission
Summary of Accounting Policies [Line Items]
Goodwill		577,000	577,000	577,000
Energy Relief Fund
Summary of Accounting Policies [Line Items]
Restricted cash		$ 41,500
Northern Pass Transmission
Summary of Accounting Policies [Line Items]
Amount of power to be purchased under capacity-related contract (up to) (in MW) | MW		1,090
Pre-tax impairment charge				318,000
After-tax impact of impairment charge				$ (204,400)
After-tax impact of impairment charge (in dollars per share) | $ / shares				$ (0.64)
Northern Pass Transmission | Electric Transmission
Summary of Accounting Policies [Line Items]
Pre-tax impairment charge				$ 239,600
Retail Non Hardship Wholesaleand Other Receivables
Summary of Accounting Policies [Line Items]
Allowance for uncollectible accounts for late fees and other receivable amounts		$ 191,300	164,100	81,500
CMA
Summary of Accounting Policies [Line Items]
Purchase price	$ 1,100,000
Target working capital amount	68,600
Goodwill	$ 52,000
The Connecticut Light and Power Company
Summary of Accounting Policies [Line Items]
Allowance for uncollectible accounts for late fees and other receivable amounts		$ 181,319	157,447	97,300
Period of accounts receivable recoverable under financial or medical duress		180 days
Maximum loans receivable outstanding		$ 55,000
Loans transferred receivable current		10,500	12,900
Loans transferred receivable non-current		8,300	9,500
Tax remittance to state for energy efficiency funds				21,400
Long term receivable		25,000	25,000
Increase to operating cash inflows in accounts payable		(24,895)	17,028	(102,344)
Increase to investing cash outflows in investments in property, plant and equipment		790,083	833,973	917,532
The Connecticut Light and Power Company | Retail Non Hardship Wholesaleand Other Receivables
Summary of Accounting Policies [Line Items]
Allowance for uncollectible accounts for late fees and other receivable amounts		$ 36,700	28,300	17,200
Yankee Gas Services Company
Summary of Accounting Policies [Line Items]
Period of accounts receivable recoverable under financial or medical duress		90 days
NSTAR Electric Company
Summary of Accounting Policies [Line Items]
Allowance for uncollectible accounts for late fees and other receivable amounts		$ 97,005	91,583	75,400
Long term receivable		5,500	5,500
Increase to operating cash inflows in accounts payable		(31,650)	(25,573)	22,659
Increase to investing cash outflows in investments in property, plant and equipment		960,949	907,000	861,391
NSTAR Electric Company | Retail Non Hardship Wholesaleand Other Receivables
Summary of Accounting Policies [Line Items]
Allowance for uncollectible accounts for late fees and other receivable amounts		53,700	51,900	31,500
Public Service Company of New Hampshire
Summary of Accounting Policies [Line Items]
Allowance for uncollectible accounts for late fees and other receivable amounts		24,331	17,157	10,500
Long term receivable		3,800	3,800
Increase to operating cash inflows in accounts payable		3,256	(27,270)	(12,281)
Increase to investing cash outflows in investments in property, plant and equipment		$ 326,379	342,586	308,993
Connecticut, Massachusetts and New Hampshire
Summary of Accounting Policies [Line Items]
Number of regulated utilities | utility		10
Restatement Adjustment
Summary of Accounting Policies [Line Items]
Allowance for uncollectible accounts for late fees and other receivable amounts			23,800
Restatement Adjustment | Retail Non Hardship Wholesaleand Other Receivables
Summary of Accounting Policies [Line Items]
Allowance for uncollectible accounts for late fees and other receivable amounts				2,200
Restatement Adjustment | The Connecticut Light and Power Company
Summary of Accounting Policies [Line Items]
Allowance for uncollectible accounts for late fees and other receivable amounts			22,200
Restatement Adjustment | The Connecticut Light and Power Company | Retail Non Hardship Wholesaleand Other Receivables
Summary of Accounting Policies [Line Items]
Allowance for uncollectible accounts for late fees and other receivable amounts				900
Restatement Adjustment | NSTAR Electric Company
Summary of Accounting Policies [Line Items]
Allowance for uncollectible accounts for late fees and other receivable amounts			(1,300)
Restatement Adjustment | NSTAR Electric Company | Retail Non Hardship Wholesaleand Other Receivables
Summary of Accounting Policies [Line Items]
Allowance for uncollectible accounts for late fees and other receivable amounts				$ 300
Restatement Adjustment | Public Service Company of New Hampshire
Summary of Accounting Policies [Line Items]
Allowance for uncollectible accounts for late fees and other receivable amounts			$ 300
COVID 19
Summary of Accounting Policies [Line Items]
Accounts receivable, allowance for uncollectible accounts increase (decrease)		$ 24,100
Allowance for uncollectible accounts for late fees and other receivable amounts		55,300
COVID 19 | The Connecticut Light and Power Company
Summary of Accounting Policies [Line Items]
Accounts receivable, allowance for uncollectible accounts increase (decrease)		20,100
Allowance for uncollectible accounts for late fees and other receivable amounts		23,900
COVID 19 | NSTAR Electric Company
Summary of Accounting Policies [Line Items]
Accounts receivable, allowance for uncollectible accounts increase (decrease)		(1,300)
Allowance for uncollectible accounts for late fees and other receivable amounts		9,000
COVID 19 | Public Service Company of New Hampshire
Summary of Accounting Policies [Line Items]
Decrease in deferral cost		600
COVID 19 | Natural Gas Businesses
Summary of Accounting Policies [Line Items]
Accounts receivable, allowance for uncollectible accounts increase (decrease)		6,600
Allowance for uncollectible accounts for late fees and other receivable amounts		$ 21,400