SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES - Narrative (Details) $ in Thousands, customer in Millions		3 Months Ended		6 Months Ended
	Feb. 26, 2020 USD ($)	Jun. 30, 2020 USD ($)	Jun. 30, 2019 USD ($)	Jun. 30, 2020 USD ($) utility customer	Jun. 30, 2019 USD ($)	Mar. 31, 2020 USD ($)	Jan. 01, 2020 USD ($)	Dec. 31, 2019 USD ($)	Mar. 31, 2019 USD ($)	Dec. 31, 2018 USD ($)
Summary of Accounting Policies [Line Items]
Number of electric, natural gas and water customers | customer				4,000,000
Allowance for uncollectible accounts for late fees and other receivable amounts		$ 270,000		$ 270,000				$ 224,800
Stockholders' equity		13,805,258	$ 11,954,593	13,805,258	$ 11,954,593	$ 13,209,956		12,629,994	$ 11,637,434	$ 11,486,817
Adoption of new accounting standard		13,805,258	11,954,593	13,805,258	11,954,593	13,209,956		12,629,994	11,637,434	11,486,817
Energy efficiency funds collected from customers to be remitted to the State of Connecticut			10,700		21,400
Columbia Gas of Massachusetts (CMA)
Summary of Accounting Policies [Line Items]
Purchase price	$ 1,100,000
Retained Earnings
Summary of Accounting Policies [Line Items]
Stockholders' equity		4,384,093	3,954,492	4,384,093	3,954,492	4,322,825		4,177,048	4,092,895	3,953,974
Adoption of new accounting standard		4,384,093	3,954,492	4,384,093	3,954,492	4,322,825		4,177,048	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		68,700		68,700				75,400
Stockholders' equity		4,160,236	3,751,354	4,160,236	3,751,354	4,063,190		4,159,884	3,783,196	3,730,155
Adoption of new accounting standard		4,160,236	3,751,354	4,160,236	3,751,354	4,063,190		4,159,884	3,783,196	3,730,155
NSTAR Electric | Retained Earnings
Summary of Accounting Policies [Line Items]
Stockholders' equity		2,346,504	2,099,059	2,346,504	2,099,059	2,249,526		2,346,287	2,131,015	2,098,091
Adoption of new accounting standard		2,346,504	2,099,059	2,346,504	2,099,059	2,249,526		2,346,287	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		136,700		136,700				97,300
Stockholders' equity		4,550,656	4,136,438	4,550,656	4,136,438	4,434,772		4,387,825	4,209,415	4,199,317
Adoption of new accounting standard		4,550,656	4,136,438	$ 4,550,656	4,136,438	4,434,772		4,387,825	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		1,954,221	1,664,995	$ 1,954,221	1,664,995	1,838,340		1,791,392	1,737,980	1,727,899
Adoption of new accounting standard		1,954,221	1,664,995	1,954,221	1,664,995	1,838,340		1,791,392	1,737,980	1,727,899
PSNH
Summary of Accounting Policies [Line Items]
Allowance for uncollectible accounts for late fees and other receivable amounts		11,800		11,800				10,500
Stockholders' equity		1,440,929	1,129,777	1,440,929	1,129,777	1,409,012		1,391,733	1,316,629	1,302,541
Adoption of new accounting standard		1,440,929	1,129,777	1,440,929	1,129,777	1,409,012		1,391,733	1,316,629	1,302,541
PSNH | Retained Earnings
Summary of Accounting Policies [Line Items]
Stockholders' equity		538,939	453,891	538,939	453,891	507,307		490,306	641,039	627,258
Adoption of new accounting standard		538,939	$ 453,891	$ 538,939	$ 453,891	$ 507,307		490,306	$ 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				8
Natural Gas Distribution | COVID-19
Summary of Accounting Policies [Line Items]
Accounts Receivable, Allowance for Credit Loss, Period Increase (Decrease)		4,100
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		94,500		$ 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		34,100		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		$ 20,400		$ 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