SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES - Narrative (Details) customer in Millions, $ in Millions	3 Months Ended	9 Months Ended
	Sep. 30, 2021 USD ($)	Sep. 30, 2021 USD ($) customer utility	Jun. 30, 2021 USD ($)	Dec. 31, 2020 USD ($)	Sep. 30, 2020 USD ($)	Jun. 30, 2020 USD ($)	Dec. 31, 2019 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	$ 441.7	$ 441.7	$ 425.8	$ 358.9	$ 295.0	$ 270.0	$ 224.8
COVID-19
Summary of Accounting Policies [Line Items]
Increase (decrease) in the allowance for uncollectible accounts	(6.5)	25.8
Allowance for uncollectible accounts for late fees and other receivable amounts	57.0	57.0
CL&P
Summary of Accounting Policies [Line Items]
Allowance for uncollectible accounts for late fees and other receivable amounts	195.5	$ 195.5	188.8	157.4	150.8	136.7	97.3
Period of accounts receivable recoverable under financial or medical duress		180 days
CL&P | COVID-19
Summary of Accounting Policies [Line Items]
Increase (decrease) in the allowance for uncollectible accounts	4.0	$ 16.3
Allowance for uncollectible accounts for late fees and other receivable amounts	20.1	20.1
NSTAR Electric
Summary of Accounting Policies [Line Items]
Allowance for uncollectible accounts for late fees and other receivable amounts	105.0	105.0	98.0	91.6	76.8	68.7	75.4
NSTAR Electric | COVID-19
Summary of Accounting Policies [Line Items]
Increase (decrease) in the allowance for uncollectible accounts	(8.3)	(1.8)
Allowance for uncollectible accounts for late fees and other receivable amounts	8.5	8.5
PSNH
Summary of Accounting Policies [Line Items]
Allowance for uncollectible accounts for late fees and other receivable amounts	17.0	17.0	$ 17.2	$ 17.2	$ 14.0	$ 11.8	$ 10.5
PSNH | COVID-19
Summary of Accounting Policies [Line Items]
Reversal of deferred net incremental COVID-19 costs		0.6
EGMA | Other Long-Term Assets
Summary of Accounting Policies [Line Items]
Restricted cash	41.5	41.5
Natural Gas Businesses | COVID-19
Summary of Accounting Policies [Line Items]
Increase (decrease) in the allowance for uncollectible accounts	(2.2)	12.5
Allowance for uncollectible accounts for late fees and other receivable amounts	$ 27.4	$ 27.4
Yankee Gas
Summary of Accounting Policies [Line Items]
Period of accounts receivable recoverable under financial or medical duress		90 days
Connecticut, Massachusetts and New Hampshire
Summary of Accounting Policies [Line Items]
Number of regulated utilities | utility		9