SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES - Narrative (Details) $ in Thousands, customer in Millions		3 Months Ended
	Feb. 26, 2020 USD ($)	Mar. 31, 2020 USD ($) customer utility	Mar. 31, 2019 USD ($)	Jan. 01, 2020 USD ($)	Dec. 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
Decrease to operating cash flows inflows in accounts payable		$ (127,366)	$ (60,121)
Decrease to investing cash outflows in investments in property, plant and equipment		(725,520)	(628,129)
Allowance for uncollectible accounts for late fees and other receivable amounts		262,100			$ 224,800
Stockholders' equity		13,209,956	11,637,434		12,629,994	$ 11,486,817
Adoption of new accounting standard				$ (1,514)
Energy efficiency funds collected from customers to be remitted to the State of Connecticut		10,700
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		4,322,825	4,092,895		4,177,048	3,953,974
Adoption of new accounting standard				(1,514)
NSTAR Electric
Summary of Accounting Policies [Line Items]
Decrease to operating cash flows inflows in accounts payable		(53,462)	(18,109)
Decrease to investing cash outflows in investments in property, plant and equipment		(247,549)	(208,540)
Allowance for uncollectible accounts for late fees and other receivable amounts		71,100			75,400
Stockholders' equity		4,063,190	3,783,196		4,159,884	3,730,155
Adoption of new accounting standard				(161)
NSTAR Electric | Retained Earnings
Summary of Accounting Policies [Line Items]
Stockholders' equity		2,249,526	2,131,015		2,346,287	2,098,091
Adoption of new accounting standard				(161)
CL&P
Summary of Accounting Policies [Line Items]
Decrease to operating cash flows inflows in accounts payable		(27,298)	75,373
Decrease to investing cash outflows in investments in property, plant and equipment		(212,374)	(189,423)
Allowance for uncollectible accounts for late fees and other receivable amounts		130,900			97,300
Stockholders' equity		$ 4,434,772	4,209,415		4,387,825	4,199,317
Adoption of new accounting standard				(900)
Period of accounts receivable recoverable under financial or medical duress		180 days
CL&P | Retained Earnings
Summary of Accounting Policies [Line Items]
Stockholders' equity		$ 1,838,340	1,737,980		1,791,392	1,727,899
Adoption of new accounting standard				(900)
PSNH
Summary of Accounting Policies [Line Items]
Decrease to operating cash flows inflows in accounts payable		(18,064)	11,762
Decrease to investing cash outflows in investments in property, plant and equipment		(88,694)	(64,361)
Allowance for uncollectible accounts for late fees and other receivable amounts		11,200			10,500
Stockholders' equity		1,409,012	1,316,629		1,391,733	1,302,541
Adoption of new accounting standard				(300)
PSNH | Retained Earnings
Summary of Accounting Policies [Line Items]
Stockholders' equity		$ 507,307	641,039		490,306	$ 627,258
Adoption of new accounting standard				(300)
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]
Decrease to operating cash flows inflows in accounts payable			46,600
Decrease to investing cash outflows in investments in property, plant and equipment			46,600
Restatement Adjustment | PSNH
Summary of Accounting Policies [Line Items]
Decrease to operating cash flows inflows in accounts payable			46,600
Decrease to investing cash outflows in investments in property, plant and equipment			$ 46,600
Connecticut, Massachusetts and New Hampshire
Summary of Accounting Policies [Line Items]
Number of regulated utilities | utility		8
Revision of Prior Period, Adjustment
Summary of Accounting Policies [Line Items]
Allowance for uncollectible accounts for late fees and other receivable amounts					23,800
Revision of Prior Period, Adjustment | NSTAR Electric
Summary of Accounting Policies [Line Items]
Allowance for uncollectible accounts for late fees and other receivable amounts					(1,300)
Revision of Prior Period, Adjustment | CL&P
Summary of Accounting Policies [Line Items]
Allowance for uncollectible accounts for late fees and other receivable amounts					22,200
Revision of Prior Period, Adjustment | PSNH
Summary of Accounting Policies [Line Items]
Allowance for uncollectible accounts for late fees and other receivable amounts					300
Adoption Adjustment | Retained Earnings
Summary of Accounting Policies [Line Items]
Stockholders' equity				(1,500)
Adoption Adjustment | NSTAR Electric | Retained Earnings
Summary of Accounting Policies [Line Items]
Stockholders' equity				(200)
Adoption Adjustment | CL&P | Retained Earnings
Summary of Accounting Policies [Line Items]
Stockholders' equity				(900)
Adoption Adjustment | PSNH
Summary of Accounting Policies [Line Items]
Allowance for uncollectible accounts for late fees and other receivable amounts				300
Adoption Adjustment | PSNH | Retained Earnings
Summary of Accounting Policies [Line Items]
Stockholders' equity				(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		$ 89,900			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		32,600			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		$ 20,200			17,200
Retail (Non-Hardship), Wholesale, and Other Receivables | Revision of Prior Period, 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 | Revision of Prior Period, 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 | Revision of Prior Period, 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 | Revision of Prior Period, 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 | Revision of Prior Period, 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 | Revision of Prior Period, Adjustment | CL&P
Summary of Accounting Policies [Line Items]
Allowance for uncollectible accounts for late fees and other receivable amounts					$ 21,300