SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES - Narrative (Details) $ in Thousands, customer in Millions		3 Months Ended	9 Months Ended
	Oct. 09, 2020 USD ($)	Sep. 30, 2020 USD ($)	Sep. 30, 2020 USD ($) customer utility	Sep. 30, 2019 USD ($)	Jun. 30, 2020 USD ($)	Mar. 31, 2020 USD ($)	Jan. 01, 2020 USD ($)	Dec. 31, 2019 USD ($)	Jun. 30, 2019 USD ($)	Mar. 31, 2019 USD ($)	Dec. 31, 2018 USD ($)
Summary of Accounting Policies [Line Items]
Number of electric, natural gas and water customers | customer			4.3
Allowance for uncollectible accounts for late fees and other receivable amounts		$ 295,000	$ 295,000		$ 270,000			$ 224,800
Stockholders' equity		13,980,017	13,980,017	$ 12,121,614	13,805,258	$ 13,209,956		12,629,994	$ 11,954,593	$ 11,637,434	$ 11,486,817
Adoption of new accounting standard		13,980,017	13,980,017	12,121,614	13,805,258	13,209,956		12,629,994	11,954,593	11,637,434	11,486,817
Energy efficiency funds collected from customers to be remitted to the State of Connecticut				21,400
CMA | Subsequent Event
Summary of Accounting Policies [Line Items]
Cash purchase price	$ 1,100,000
Retained Earnings
Summary of Accounting Policies [Line Items]
Stockholders' equity		4,535,860	4,535,860	4,100,220	4,384,093	4,322,825		4,177,048	3,954,492	4,092,895	3,953,974
Adoption of new accounting standard		4,535,860	4,535,860	4,100,220	4,384,093	4,322,825		4,177,048	3,954,492	4,092,895	3,953,974
NSTAR Electric
Summary of Accounting Policies [Line Items]
Allowance for uncollectible accounts for late fees and other receivable amounts		76,800	76,800		68,700			75,400
Stockholders' equity		4,325,539	4,325,539	3,915,916	4,160,236	4,063,190		4,159,884	3,751,354	3,783,196	3,730,155
Adoption of new accounting standard		4,325,539	4,325,539	3,915,916	4,160,236	4,063,190		4,159,884	3,751,354	3,783,196	3,730,155
NSTAR Electric | Retained Earnings
Summary of Accounting Policies [Line Items]
Stockholders' equity		2,511,742	2,511,742	2,253,509	2,346,504	2,249,526		2,346,287	2,099,059	2,131,015	2,098,091
Adoption of new accounting standard		2,511,742	2,511,742	2,253,509	2,346,504	2,249,526		2,346,287	2,099,059	2,131,015	2,098,091
CL&P
Summary of Accounting Policies [Line Items]
Allowance for uncollectible accounts for late fees and other receivable amounts		150,800	150,800		136,700			97,300
Stockholders' equity		4,794,352	4,794,352	4,180,334	4,550,656	4,434,772		4,387,825	4,136,438	4,209,415	4,199,317
Adoption of new accounting standard		4,794,352	$ 4,794,352	4,180,334	4,550,656	4,434,772		4,387,825	4,136,438	4,209,415	4,199,317
Period of accounts receivable recoverable under financial or medical duress			180 days
CL&P | Retained Earnings
Summary of Accounting Policies [Line Items]
Stockholders' equity		2,072,928	$ 2,072,928	1,708,892	1,954,221	1,838,340		1,791,392	1,664,995	1,737,980	1,727,899
Adoption of new accounting standard		2,072,928	2,072,928	1,708,892	1,954,221	1,838,340		1,791,392	1,664,995	1,737,980	1,727,899
PSNH
Summary of Accounting Policies [Line Items]
Allowance for uncollectible accounts for late fees and other receivable amounts		14,000	14,000		11,800			10,500
Stockholders' equity		1,488,340	1,488,340	1,151,934	1,440,929	1,409,012		1,391,733	1,129,777	1,316,629	1,302,541
Adoption of new accounting standard		1,488,340	1,488,340	1,151,934	1,440,929	1,409,012		1,391,733	1,129,777	1,316,629	1,302,541
PSNH | Retained Earnings
Summary of Accounting Policies [Line Items]
Stockholders' equity		586,090	586,090	475,770	538,939	507,307		490,306	453,891	641,039	627,258
Adoption of new accounting standard		586,090	$ 586,090	$ 475,770	538,939	$ 507,307		490,306	$ 453,891	$ 641,039	$ 627,258
Yankee Gas
Summary of Accounting Policies [Line Items]
Period of accounts receivable recoverable under financial or medical duress			90 days
Restatement Adjustment
Summary of Accounting Policies [Line Items]
Allowance for uncollectible accounts for late fees and other receivable amounts								23,800
Restatement Adjustment | NSTAR Electric
Summary of Accounting Policies [Line Items]
Allowance for uncollectible accounts for late fees and other receivable amounts								(1,300)
Restatement Adjustment | CL&P
Summary of Accounting Policies [Line Items]
Allowance for uncollectible accounts for late fees and other receivable amounts								22,200
Restatement Adjustment | PSNH
Summary of Accounting Policies [Line Items]
Allowance for uncollectible accounts for late fees and other receivable amounts								300
Connecticut, Massachusetts and New Hampshire
Summary of Accounting Policies [Line Items]
Number of regulated utilities | utility			9
COVID-19
Summary of Accounting Policies [Line Items]
Increase in the allowance for uncollectible accounts		11,300	$ 16,500
COVID-19 | NSTAR Electric
Summary of Accounting Policies [Line Items]
Increase in the allowance for uncollectible accounts		5,100	5,100
COVID-19 | CL&P
Summary of Accounting Policies [Line Items]
Increase in the allowance for uncollectible accounts		2,800	2,800
COVID-19 | PSNH
Summary of Accounting Policies [Line Items]
Increase in the allowance for uncollectible accounts		1,500	1,500
Adoption Adjustment
Summary of Accounting Policies [Line Items]
Stockholders' equity								(1,514)
Adoption of new accounting standard								(1,514)
Adoption Adjustment | Retained Earnings
Summary of Accounting Policies [Line Items]
Stockholders' equity							$ (1,500)	(1,514)
Adoption of new accounting standard							(1,500)	(1,514)
Adoption Adjustment | NSTAR Electric
Summary of Accounting Policies [Line Items]
Stockholders' equity								(161)
Adoption of new accounting standard								(161)
Adoption Adjustment | NSTAR Electric | Retained Earnings
Summary of Accounting Policies [Line Items]
Stockholders' equity							(200)	(161)
Adoption of new accounting standard							(200)	(161)
Adoption Adjustment | CL&P
Summary of Accounting Policies [Line Items]
Stockholders' equity								(900)
Adoption of new accounting standard								(900)
Adoption Adjustment | CL&P | Retained Earnings
Summary of Accounting Policies [Line Items]
Stockholders' equity							(900)	(900)
Adoption of new accounting standard							(900)	(900)
Adoption Adjustment | PSNH
Summary of Accounting Policies [Line Items]
Allowance for uncollectible accounts for late fees and other receivable amounts							300
Stockholders' equity								(300)
Adoption of new accounting standard								(300)
Adoption Adjustment | PSNH | Retained Earnings
Summary of Accounting Policies [Line Items]
Stockholders' equity							(300)	(300)
Adoption of new accounting standard							(300)	(300)
Retail (Non-Hardship), Wholesale, and Other Receivables
Summary of Accounting Policies [Line Items]
Allowance for uncollectible accounts for late fees and other receivable amounts		116,300	116,300		94,500			81,500
Retail (Non-Hardship), Wholesale, and Other Receivables | NSTAR Electric
Summary of Accounting Policies [Line Items]
Allowance for uncollectible accounts for late fees and other receivable amounts		43,700	43,700		34,100			31,500
Retail (Non-Hardship), Wholesale, and Other Receivables | CL&P
Summary of Accounting Policies [Line Items]
Allowance for uncollectible accounts for late fees and other receivable amounts		$ 25,600	$ 25,600		$ 20,400			17,200
Retail (Non-Hardship), Wholesale, and Other Receivables | Restatement Adjustment
Summary of Accounting Policies [Line Items]
Allowance for uncollectible accounts for late fees and other receivable amounts								2,200
Retail (Non-Hardship), Wholesale, and Other Receivables | Restatement Adjustment | NSTAR Electric
Summary of Accounting Policies [Line Items]
Allowance for uncollectible accounts for late fees and other receivable amounts								300
Retail (Non-Hardship), Wholesale, and Other Receivables | Restatement Adjustment | CL&P
Summary of Accounting Policies [Line Items]
Allowance for uncollectible accounts for late fees and other receivable amounts								900
Retail (Non-Hardship), Wholesale, and Other Receivables | Adoption Adjustment
Summary of Accounting Policies [Line Items]
Allowance for uncollectible accounts for late fees and other receivable amounts							1,600
Retail (Non-Hardship), Wholesale, and Other Receivables | Adoption Adjustment | NSTAR Electric
Summary of Accounting Policies [Line Items]
Allowance for uncollectible accounts for late fees and other receivable amounts							200
Retail (Non-Hardship), Wholesale, and Other Receivables | Adoption Adjustment | CL&P
Summary of Accounting Policies [Line Items]
Allowance for uncollectible accounts for late fees and other receivable amounts							$ 900
Hardship Accounts and Other Low-Income Assistance Programs | Restatement Adjustment
Summary of Accounting Policies [Line Items]
Allowance for uncollectible accounts for late fees and other receivable amounts								22,200
Hardship Accounts and Other Low-Income Assistance Programs | Restatement Adjustment | NSTAR Electric
Summary of Accounting Policies [Line Items]
Allowance for uncollectible accounts for late fees and other receivable amounts								(1,500)
Hardship Accounts and Other Low-Income Assistance Programs | Restatement Adjustment | CL&P
Summary of Accounting Policies [Line Items]
Allowance for uncollectible accounts for late fees and other receivable amounts								$ 21,300