EMPLOYEE BENEFITS - Share-Based Payments Narrative (Details) - USD ($) $ / shares in Units, $ in Millions	12 Months Ended
Dec. 31, 2021	Dec. 31, 2020	Dec. 31, 2019
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Shares granted in incentive plan (up to) (in shares)	6,700,000
Shares available for issuance under incentive plan (in shares)	2,430,716	2,876,601
Shares outstanding (in shares)	297,270
Weighted average grant date fair value of restricted stock units (in dollars per share)	$ 72.37
Total unrecognized compensation expense	$ 17.8
Period for recognition	1 year 8 months 19 days
Tax effect on total share-based payments (in percentage)	26.00%
Excess stock benefit	$ 4.0	$ 6.6	$ 1.5
Restricted Stock Units (RSUs)
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Granted (in dollars per share)	$ 81.89	$ 88.23	$ 67.91
Shares outstanding (in shares)		379,258
Weighted average shares outstanding (in dollars per share)	$ 83.39	$ 77.13
Restricted stock units available (in shares)	219,560
Restricted stock unit fully vested (in shares)	297,353
Additional restricted stock unit expected to vest (in shares)	282,407
Performance Shares
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Granted (in dollars per share)	$ 76.08	$ 75.36	$ 68.33
Shares outstanding (in shares)	436,957	404,698
Weighted average shares outstanding (in dollars per share)	$ 81.41	$ 70.85
Restricted stock units available (in shares)	241,949
Weighted average grant date fair value of restricted stock units (in dollars per share)	$ 57.23
Restricted stock unit fully vested (in shares)	27,550
The Connecticut Light and Power Company
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Total unrecognized compensation expense	$ 3.2
Excess stock benefit	1.5	$ 2.3	$ 0.5
NSTAR Electric Company
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Total unrecognized compensation expense	5.0
Excess stock benefit	1.4	2.3	0.5
Public Service Company of New Hampshire
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Total unrecognized compensation expense	1.1
Excess stock benefit	$ 0.5	$ 0.8	$ 0.2
Employee
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Period of graded vesting schedule under long-term incentive program	3 years
Board Members
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Period of graded vesting schedule under long-term incentive program	1 year

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Amount of cost not yet recognized for nonvested award under share-based payment arrangement.

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Weighted-average period over which cost not yet recognized is expected to be recognized for award under share-based payment arrangement, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents reported fact of one year, five months, and thirteen days.

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The weighted average fair value at grant date for nonvested equity-based awards issued during the period on other than stock (or unit) option plans (for example, phantom stock or unit plan, stock or unit appreciation rights plan, performance target plan).

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The number of non-vested equity-based payment instruments, excluding stock (or unit) options, that validly exist and are outstanding as of the balance sheet date.

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Per share or unit weighted-average fair value of nonvested award under share-based payment arrangement. Excludes share and unit options.

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