SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES - Narrative (Details) $ / shares in Units, $ in Thousands, customer in Millions		12 Months Ended
	Oct. 09, 2020 USD ($)	Dec. 31, 2020 USD ($) customer utility MW	Dec. 31, 2019 USD ($) $ / shares	Dec. 31, 2018 USD ($)	Jan. 01, 2020 USD ($)	Dec. 31, 2017 USD ($)
Summary of Accounting Policies [Line Items]
Number of electric and natural gas customers | customer		4.3
Goodwill		$ 4,445,988	$ 4,427,266
Allowance for uncollectible accounts for late fees and other receivable amounts		$ 358,851	224,821
Accounting Standards Update [Extensible List]		us-gaap:AccountingStandardsUpdate201613Member
Stockholders' equity		$ 14,063,566	12,629,994	$ 11,486,817		$ 11,086,242
Pre-tax impairment charge		0	239,644	0
After-tax impact of impairment charge			$ 204,400
After-tax impact of impairment charge (in dollars per share) | $ / shares			$ 0.64
Increase (decrease) in receivables		206,500
Operating lease liabilities		55,200	$ 49,900
Charitable contributions		6,400	0	0
Increase to operating cash inflows in accounts payable		(122,567)	(14,866)	(24,481)
Increase to investing cash outflows in investments in property, plant and equipment		2,942,996	2,911,489	2,569,936
Energy Relief Fund
Summary of Accounting Policies [Line Items]
Restricted cash		41,500
Retained Earnings
Summary of Accounting Policies [Line Items]
Stockholders' equity		4,613,201	4,177,048	3,953,974		3,561,084
Retail Non Hardship Wholesaleand Other Receivables
Summary of Accounting Policies [Line Items]
Allowance for uncollectible accounts for late fees and other receivable amounts		164,100	81,500
CMA
Summary of Accounting Policies [Line Items]
Purchase price	$ 1,100,000
Goodwill	42,000
Target working capital amount	$ 69,600
Adoption of New Accounting Standard
Summary of Accounting Policies [Line Items]
Stockholders' equity			(1,514)
Adoption of New Accounting Standard | Retained Earnings
Summary of Accounting Policies [Line Items]
Stockholders' equity			(1,514)		$ 1,500
Adoption of New Accounting Standard | Retail Non Hardship Wholesaleand Other Receivables
Summary of Accounting Policies [Line Items]
Allowance for uncollectible accounts for late fees and other receivable amounts					1,600
The Connecticut Light and Power Company
Summary of Accounting Policies [Line Items]
Allowance for uncollectible accounts for late fees and other receivable amounts		$ 157,447	97,348
Accounting Standards Update [Extensible List]		us-gaap:AccountingStandardsUpdate201613Member
Stockholders' equity		$ 5,044,786	4,387,825	4,199,317		3,587,127
Increase (decrease) in receivables		58,300
Operating lease liabilities		$ 300	700
Period of accounts receivable recoverable under financial or medical duress		180 days
Maximum loans receivable outstanding		$ 55,000
Loans transferred receivable current		12,900	16,500
Loans transferred receivable non-current		9,500	18,200
Tax remittance to state for energy efficiency funds			21,400	46,800
Long term receivable		25,000	25,000
Increase to operating cash inflows in accounts payable		17,028	(102,344)	76,924
Increase to investing cash outflows in investments in property, plant and equipment		833,973	917,532	864,136
The Connecticut Light and Power Company | Retained Earnings
Summary of Accounting Policies [Line Items]
Stockholders' equity		2,173,367	1,791,392	1,727,899		1,415,741
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		$ 28,300	17,200
The Connecticut Light and Power Company | Adoption of New Accounting Standard
Summary of Accounting Policies [Line Items]
Stockholders' equity			(900)
The Connecticut Light and Power Company | Adoption of New Accounting Standard | Retained Earnings
Summary of Accounting Policies [Line Items]
Stockholders' equity			(900)		900
The Connecticut Light and Power Company | Adoption of New Accounting Standard | Retail Non Hardship Wholesaleand Other Receivables
Summary of Accounting Policies [Line Items]
Allowance for uncollectible accounts for late fees and other receivable amounts					900
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		$ 91,583	75,406
Accounting Standards Update [Extensible List]		us-gaap:AccountingStandardsUpdate201613Member
Stockholders' equity		$ 4,521,418	4,159,884	3,730,155		3,446,080
Increase (decrease) in receivables		56,300
Operating lease liabilities		23,600	24,200
Long term receivable		5,500	5,500
Increase to operating cash inflows in accounts payable		(25,573)	22,659	(37,140)
Increase to investing cash outflows in investments in property, plant and equipment		907,000	861,391	725,766
NSTAR Electric Company | Retained Earnings
Summary of Accounting Policies [Line Items]
Stockholders' equity		2,527,167	2,346,287	2,098,091		1,944,961
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		51,900	31,500
NSTAR Electric Company | Adoption of New Accounting Standard
Summary of Accounting Policies [Line Items]
Stockholders' equity			(161)
NSTAR Electric Company | Adoption of New Accounting Standard | Retained Earnings
Summary of Accounting Policies [Line Items]
Stockholders' equity			(161)		200
NSTAR Electric Company | Adoption of New Accounting Standard | Retail Non Hardship Wholesaleand Other Receivables
Summary of Accounting Policies [Line Items]
Allowance for uncollectible accounts for late fees and other receivable amounts					200
Public Service Company of New Hampshire
Summary of Accounting Policies [Line Items]
Allowance for uncollectible accounts for late fees and other receivable amounts		$ 17,157	10,497
Accounting Standards Update [Extensible List]		us-gaap:AccountingStandardsUpdate201613Member
Stockholders' equity		$ 1,542,539	1,391,733	1,302,541		1,350,594
Increase (decrease) in receivables		20,000
Operating lease liabilities		300	400
Long term receivable		3,800	3,800
Increase to operating cash inflows in accounts payable		(27,270)	(12,281)	10,147
Increase to investing cash outflows in investments in property, plant and equipment		342,586	308,993	323,910
Public Service Company of New Hampshire | Retained Earnings
Summary of Accounting Policies [Line Items]
Stockholders' equity		615,018	490,306	$ 627,258		$ 511,382
Public Service Company of New Hampshire | Adoption of New Accounting Standard
Summary of Accounting Policies [Line Items]
Stockholders' equity			(300)
Public Service Company of New Hampshire | Adoption of New Accounting Standard | Retained Earnings
Summary of Accounting Policies [Line Items]
Stockholders' equity			(300)		300
Public Service Company of New Hampshire | Adoption of New Accounting Standard | Retail Non Hardship Wholesaleand Other Receivables
Summary of Accounting Policies [Line Items]
Allowance for uncollectible accounts for late fees and other receivable amounts					$ 300
EGMA
Summary of Accounting Policies [Line Items]
Increase (decrease) in receivables		$ 65,800
Connecticut, Massachusetts and New Hampshire
Summary of Accounting Policies [Line Items]
Number of regulated utilities | utility		9
Northern Pass
Summary of Accounting Policies [Line Items]
Capacity required for transmission line | MW		1,090
Capitalized costs			318,000
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 | Hardship Accounts and Other Low Income Assistance Programs
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
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 | The Connecticut Light and Power Company | Hardship Accounts and Other Low Income Assistance Programs
Summary of Accounting Policies [Line Items]
Allowance for uncollectible accounts for late fees and other receivable amounts		21,300
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 | NSTAR Electric Company | Hardship Accounts and Other Low Income Assistance Programs
Summary of Accounting Policies [Line Items]
Allowance for uncollectible accounts for late fees and other receivable amounts		1,500
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		31,500
COVID 19 | The Connecticut Light and Power Company
Summary of Accounting Policies [Line Items]
Accounts receivable		2,800
COVID 19 | NSTAR Electric Company
Summary of Accounting Policies [Line Items]
Accounts receivable		11,000
COVID 19 | Public Service Company of New Hampshire
Summary of Accounting Policies [Line Items]
Accounts receivable		2,300
COVID 19 | Natural Gas Businesses
Summary of Accounting Policies [Line Items]
Accounts receivable		$ 15,400