SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES - Narrative (Details) customer in Millions, $ in Millions	3 Months Ended	6 Months Ended
	Jun. 30, 2021 USD ($)	Jun. 30, 2021 USD ($) utility customer	Mar. 31, 2021 USD ($)	Dec. 31, 2020 USD ($)	Jun. 30, 2020 USD ($)	Mar. 31, 2020 USD ($)	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	$ 425.8	$ 425.8	$ 391.7	$ 358.9	$ 270.0	$ 262.1	$ 224.8
COVID-19
Summary of Accounting Policies [Line Items]
Increase in the allowance for uncollectible accounts		32.1
Allowance for uncollectible accounts for late fees and other receivable amounts	63.6	63.6
CL&P
Summary of Accounting Policies [Line Items]
Allowance for uncollectible accounts for late fees and other receivable amounts	188.8	$ 188.8	175.1	157.4	136.7	130.9	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 in the allowance for uncollectible accounts		$ 12.3
Allowance for uncollectible accounts for late fees and other receivable amounts	15.1	15.1
NSTAR Electric
Summary of Accounting Policies [Line Items]
Allowance for uncollectible accounts for late fees and other receivable amounts	98.0	98.0	89.3	91.6	68.7	71.1	75.4
NSTAR Electric | COVID-19
Summary of Accounting Policies [Line Items]
Increase in the allowance for uncollectible accounts		6.3
Allowance for uncollectible accounts for late fees and other receivable amounts	17.3	17.3
PSNH
Summary of Accounting Policies [Line Items]
Allowance for uncollectible accounts for late fees and other receivable amounts	17.2	17.2	$ 17.3	$ 17.2	$ 11.8	$ 11.2	$ 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 in the allowance for uncollectible accounts		14.7
Allowance for uncollectible accounts for late fees and other receivable amounts	$ 30.1	$ 30.1
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