SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES - Narrative (Details) customer in Millions, $ in Millions	3 Months Ended
	Mar. 31, 2021 USD ($) customer utility	Dec. 31, 2020 USD ($)	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	$ 391.7	$ 358.9	$ 262.1	$ 224.8
COVID-19
Summary of Accounting Policies [Line Items]
Increase in the allowance for uncollectible accounts	15.5
Allowance for uncollectible accounts for late fees and other receivable amounts	47.0
CL&P
Summary of Accounting Policies [Line Items]
Allowance for uncollectible accounts for late fees and other receivable amounts	$ 175.1	157.4	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	$ 5.6
Allowance for uncollectible accounts for late fees and other receivable amounts	8.4
NSTAR Electric
Summary of Accounting Policies [Line Items]
Allowance for uncollectible accounts for late fees and other receivable amounts	89.3	91.6	71.1	75.4
NSTAR Electric | COVID-19
Summary of Accounting Policies [Line Items]
Increase in the allowance for uncollectible accounts	3.7
Allowance for uncollectible accounts for late fees and other receivable amounts	14.7
PSNH
Summary of Accounting Policies [Line Items]
Allowance for uncollectible accounts for late fees and other receivable amounts	17.3	$ 17.2	$ 11.2	$ 10.5
PSNH | COVID-19
Summary of Accounting Policies [Line Items]
Allowance for uncollectible accounts for late fees and other receivable amounts	2.3
EGMA | Other Long-Term Assets
Summary of Accounting Policies [Line Items]
Restricted cash	41.5
Natural Gas Businesses | COVID-19
Summary of Accounting Policies [Line Items]
Increase in the allowance for uncollectible accounts	6.2
Allowance for uncollectible accounts for late fees and other receivable amounts	$ 21.6
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