EMPLOYEE BENEFITS - Share-Based Payments Narrative (Details) - USD ($) $ / shares in Units, $ in Millions	12 Months Ended
	Dec. 31, 2023	Dec. 31, 2022	Dec. 31, 2021
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Shares granted in incentive plan (up to) (in shares)	7,400,000
Shares available for issuance under incentive plan (in shares)	4,587,376	903,183
Shares outstanding (in shares)	326,581
Weighted average grant date fair value of restricted stock units (in dollars per share)	$ 86.92
Total unrecognized compensation expense	$ 31.3
Period for recognition	1 year 9 months 21 days
Tax effect on total share-based payments (in percentage)	26.00%
Excess stock benefit	$ (0.5)	$ 2.1	$ 4.0
Restricted Stock Units (RSUs)
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Granted (in dollars per share)	$ 76.42	$ 85.96	$ 81.89
Shares outstanding (in shares)		300,592
Weighted average shares outstanding (in dollars per share)	$ 80.76	$ 87.21
Restricted stock units available (in shares)	199,145
Restricted stock unit fully vested (in shares)	345,661
Additional restricted stock unit expected to vest (in shares)	310,252
Performance Shares
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Granted (in dollars per share)	$ 83.39	$ 83.34	$ 76.08
Shares outstanding (in shares)	485,480	457,069
Weighted average shares outstanding (in dollars per share)	$ 85.20	$ 88.43
Restricted stock units available (in shares)	214,742
Weighted average grant date fair value of restricted stock units (in dollars per share)	$ 89.70
Restricted stock unit fully vested (in shares)	178,944
CL&P
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Total unrecognized compensation expense	$ 5.8
Excess stock benefit	(0.2)	$ 0.7	$ 1.5
NSTAR Electric
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Total unrecognized compensation expense	8.6
Excess stock benefit	(0.2)	0.8	1.4
PSNH
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Total unrecognized compensation expense	1.9
Excess stock benefit	$ (0.1)	$ 0.3	$ 0.5
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