SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES - Other Income (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2023	Dec. 31, 2022	Dec. 31, 2021
Summary of Accounting Policies [Line Items]
Other Income, Net	$ 348,069	$ 346,088	$ 161,282
Unrealized gain associated with investment	15,500	22,900	14,200
Pension, SERP and PBOP Non-Service Income Components, Net of Deferred Portion
Summary of Accounting Policies [Line Items]
Other Income, Net	132,900	219,800	84,400
AFUDC Equity
Summary of Accounting Policies [Line Items]
Other Income, Net	78,100	47,300	37,300
Equity in Earnings of Unconsolidated Affiliates
Summary of Accounting Policies [Line Items]
Other Income, Net	15,500	22,900	14,200
Unrealized gain associated with investment		12,200	2,100
Investment (Loss)/Income
Summary of Accounting Policies [Line Items]
Other Income, Net	(4,900)	1,900	(200)
Interest Income
Summary of Accounting Policies [Line Items]
Other Income, Net	94,200	50,500	25,600
Other
Summary of Accounting Policies [Line Items]
Other Income, Net	32,300	3,700	0
CL&P
Summary of Accounting Policies [Line Items]
Other Income, Net	61,560	83,252	30,187
CL&P | Pension, SERP and PBOP Non-Service Income Components, Net of Deferred Portion
Summary of Accounting Policies [Line Items]
Other Income, Net	34,900	64,400	15,200
CL&P | AFUDC Equity
Summary of Accounting Policies [Line Items]
Other Income, Net	20,000	13,600	7,700
CL&P | Investment (Loss)/Income
Summary of Accounting Policies [Line Items]
Other Income, Net	(2,400)	(1,300)	1,300
CL&P | Interest Income
Summary of Accounting Policies [Line Items]
Other Income, Net	9,000	6,500	5,900
CL&P | Other
Summary of Accounting Policies [Line Items]
Other Income, Net	100	100	100
NSTAR Electric
Summary of Accounting Policies [Line Items]
Other Income, Net	164,129	142,661	74,844
NSTAR Electric | Pension, SERP and PBOP Non-Service Income Components, Net of Deferred Portion
Summary of Accounting Policies [Line Items]
Other Income, Net	57,400	85,500	40,200
NSTAR Electric | AFUDC Equity
Summary of Accounting Policies [Line Items]
Other Income, Net	45,700	24,600	20,400
NSTAR Electric | Investment (Loss)/Income
Summary of Accounting Policies [Line Items]
Other Income, Net	(200)	1,200	100
NSTAR Electric | Interest Income
Summary of Accounting Policies [Line Items]
Other Income, Net	60,600	30,700	13,400
NSTAR Electric | Other
Summary of Accounting Policies [Line Items]
Other Income, Net	600	700	700
PSNH
Summary of Accounting Policies [Line Items]
Other Income, Net	26,597	32,666	14,565
PSNH | Pension, SERP and PBOP Non-Service Income Components, Net of Deferred Portion
Summary of Accounting Policies [Line Items]
Other Income, Net	16,200	26,800	10,300
PSNH | AFUDC Equity
Summary of Accounting Policies [Line Items]
Other Income, Net	5,400	2,500	1,600
PSNH | Investment (Loss)/Income
Summary of Accounting Policies [Line Items]
Other Income, Net	(700)	200	100
PSNH | Interest Income
Summary of Accounting Policies [Line Items]
Other Income, Net	5,300	3,100	2,400
PSNH | Other
Summary of Accounting Policies [Line Items]
Other Income, Net	$ 400	$ 100	$ 200