SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES - Components of Other Income, Net (Details) - USD ($)	3 Months Ended		9 Months Ended
	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019
Summary of Accounting Policies [Line Items]
Other income, net	$ 29,218,000	$ 26,968,000	$ 83,565,000	$ 103,818,000
Pension, SERP and PBOP Non-Service Income Components
Summary of Accounting Policies [Line Items]
Other income, net	11,000,000.0	10,100,000	34,300,000	23,200,000
AFUDC Equity
Summary of Accounting Policies [Line Items]
Other income, net	10,300,000	10,500,000	31,800,000	34,500,000
Equity in Earnings of Unconsolidated Affiliates
Summary of Accounting Policies [Line Items]
Other income, net	3,900,000	6,100,000	13,800,000	37,000,000.0
Unrealized gain associated with investment		0	2,400,000	20,400,000
Investment Income/(Loss)
Summary of Accounting Policies [Line Items]
Other income, net	1,000,000.0	(2,800,000)	(1,500,000)	(2,300,000)
Interest Income
Summary of Accounting Policies [Line Items]
Other income, net	1,500,000	2,900,000	3,300,000	10,800,000
Gain on Sale of Property
Summary of Accounting Policies [Line Items]
Other income, net	1,500,000	0	1,800,000	200,000
Other
Summary of Accounting Policies [Line Items]
Other income, net	0	200,000	100,000	400,000
CL&P
Summary of Accounting Policies [Line Items]
Other income, net	5,917,000	4,831,000	16,273,000	11,564,000
CL&P | Pension, SERP and PBOP Non-Service Income Components
Summary of Accounting Policies [Line Items]
Other income, net	900,000	1,700,000	3,000,000.0	100,000
CL&P | AFUDC Equity
Summary of Accounting Policies [Line Items]
Other income, net	3,500,000	3,600,000	11,400,000	9,400,000
CL&P | Equity in Earnings of Unconsolidated Affiliates
Summary of Accounting Policies [Line Items]
Other income, net	0	0	0	200,000
CL&P | Investment Income/(Loss)
Summary of Accounting Policies [Line Items]
Other income, net	1,300,000	(800,000)	0	900,000
CL&P | Interest Income
Summary of Accounting Policies [Line Items]
Other income, net	200,000	300,000	1,800,000	1,100,000
CL&P | Gain on Sale of Property
Summary of Accounting Policies [Line Items]
Other income, net	0	0	0	0
CL&P | Other
Summary of Accounting Policies [Line Items]
Other income, net	0	0	100,000	(100,000)
NSTAR Electric
Summary of Accounting Policies [Line Items]
Other income, net	12,802,000	10,735,000	38,152,000	32,479,000
NSTAR Electric | Pension, SERP and PBOP Non-Service Income Components
Summary of Accounting Policies [Line Items]
Other income, net	7,200,000	5,600,000	22,200,000	18,000,000.0
NSTAR Electric | AFUDC Equity
Summary of Accounting Policies [Line Items]
Other income, net	5,500,000	5,600,000	15,800,000	14,700,000
NSTAR Electric | Equity in Earnings of Unconsolidated Affiliates
Summary of Accounting Policies [Line Items]
Other income, net	100,000	200,000	400,000	600,000
NSTAR Electric | Investment Income/(Loss)
Summary of Accounting Policies [Line Items]
Other income, net	(200,000)	(900,000)	(1,500,000)	(1,500,000)
NSTAR Electric | Interest Income
Summary of Accounting Policies [Line Items]
Other income, net	100,000	100,000	700,000	500,000
NSTAR Electric | Gain on Sale of Property
Summary of Accounting Policies [Line Items]
Other income, net	0	0	300,000	0
NSTAR Electric | Other
Summary of Accounting Policies [Line Items]
Other income, net	100,000	100,000	300,000	200,000
PSNH
Summary of Accounting Policies [Line Items]
Other income, net	4,065,000	4,634,000	10,882,000	14,640,000
PSNH | Pension, SERP and PBOP Non-Service Income Components
Summary of Accounting Policies [Line Items]
Other income, net	1,800,000	1,400,000	5,200,000	3,500,000
PSNH | AFUDC Equity
Summary of Accounting Policies [Line Items]
Other income, net	900,000	1,000,000.0	3,500,000	2,100,000
PSNH | Equity in Earnings of Unconsolidated Affiliates
Summary of Accounting Policies [Line Items]
Other income, net	0	0	0	0
PSNH | Investment Income/(Loss)
Summary of Accounting Policies [Line Items]
Other income, net	200,000	(300,000)	100,000	0
PSNH | Interest Income
Summary of Accounting Policies [Line Items]
Other income, net	1,200,000	2,500,000	2,000,000.0	9,000,000.0
PSNH | Gain on Sale of Property
Summary of Accounting Policies [Line Items]
Other income, net	0	0	0	0
PSNH | Other
Summary of Accounting Policies [Line Items]
Other income, net	$ 0	$ 0	$ 100,000	$ 0