SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES - Other Income (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Summary of Accounting Policies [Line Items]
Other Income, Net	$ 16,718	$ 28,536	$ 100,656	$ 79,178
Equity securities, unrealized gain (loss)	17,600	9,700	0	0
Pension, SERP and PBOP Non-Service Income Components
Summary of Accounting Policies [Line Items]
Other Income, Net	14,800	7,300	44,600	22,900
AFUDC Equity
Summary of Accounting Policies [Line Items]
Other Income, Net	12,000	9,200	32,600	23,800
Equity in Earnings/(Loss) and Impairment of Unconsolidated Affiliates
Summary of Accounting Policies [Line Items]
Other Income, Net	(27,900)	5,100	(400)	23,000
Investment Income
Summary of Accounting Policies [Line Items]
Other Income, Net	1,800	4,600	2,200	3,700
Interest Income
Summary of Accounting Policies [Line Items]
Other Income, Net	10,800	2,300	16,200	5,800
Gain on Sale of Property
Summary of Accounting Policies [Line Items]
Other Income, Net	5,000	0	5,000	0
Other
Summary of Accounting Policies [Line Items]
Other Income, Net	200	0	500	0
The Connecticut Light and Power Company
Summary of Accounting Policies [Line Items]
Other Income, Net	7,098	7,913	20,722	15,402
The Connecticut Light and Power Company | Pension, SERP and PBOP Non-Service Income Components
Summary of Accounting Policies [Line Items]
Other Income, Net	2,000	400	7,300	1,300
The Connecticut Light and Power Company | AFUDC Equity
Summary of Accounting Policies [Line Items]
Other Income, Net	3,300	3,300	9,400	8,200
The Connecticut Light and Power Company | Equity in Earnings/(Loss) and Impairment of Unconsolidated Affiliates
Summary of Accounting Policies [Line Items]
Other Income, Net	0	0	0	0
The Connecticut Light and Power Company | Investment Income
Summary of Accounting Policies [Line Items]
Other Income, Net	700	3,100	900	2,400
The Connecticut Light and Power Company | Interest Income
Summary of Accounting Policies [Line Items]
Other Income, Net	900	1,100	2,900	3,500
The Connecticut Light and Power Company | Gain on Sale of Property
Summary of Accounting Policies [Line Items]
Other Income, Net		0		0
The Connecticut Light and Power Company | Other
Summary of Accounting Policies [Line Items]
Other Income, Net	200	0	200	0
NSTAR Electric Company
Summary of Accounting Policies [Line Items]
Other Income, Net	13,697	9,165	40,567	24,610
NSTAR Electric Company | Pension, SERP and PBOP Non-Service Income Components
Summary of Accounting Policies [Line Items]
Other Income, Net	9,000	4,700	26,800	14,500
NSTAR Electric Company | AFUDC Equity
Summary of Accounting Policies [Line Items]
Other Income, Net	4,200	2,700	11,500	6,700
NSTAR Electric Company | Equity in Earnings/(Loss) and Impairment of Unconsolidated Affiliates
Summary of Accounting Policies [Line Items]
Other Income, Net	200	300	600	200
NSTAR Electric Company | Investment Income
Summary of Accounting Policies [Line Items]
Other Income, Net	(400)	900	600	2,100
NSTAR Electric Company | Interest Income
Summary of Accounting Policies [Line Items]
Other Income, Net	200	600	600	1,100
NSTAR Electric Company | Gain on Sale of Property
Summary of Accounting Policies [Line Items]
Other Income, Net	500	0	500	0
NSTAR Electric Company | Other
Summary of Accounting Policies [Line Items]
Other Income, Net	0	0	0	0
Public Service Company of New Hampshire
Summary of Accounting Policies [Line Items]
Other Income, Net	16,095	2,664	24,253	7,367
Public Service Company of New Hampshire | Pension, SERP and PBOP Non-Service Income Components
Summary of Accounting Policies [Line Items]
Other Income, Net	2,000	1,500	6,400	4,500
Public Service Company of New Hampshire | AFUDC Equity
Summary of Accounting Policies [Line Items]
Other Income, Net	0	0	0	0
Public Service Company of New Hampshire | Equity in Earnings/(Loss) and Impairment of Unconsolidated Affiliates
Summary of Accounting Policies [Line Items]
Other Income, Net	0	0	0	0
Public Service Company of New Hampshire | Investment Income
Summary of Accounting Policies [Line Items]
Other Income, Net	100	700	200	1,200
Public Service Company of New Hampshire | Interest Income
Summary of Accounting Policies [Line Items]
Other Income, Net	9,600	500	13,300	1,600
Public Service Company of New Hampshire | Gain on Sale of Property
Summary of Accounting Policies [Line Items]
Other Income, Net	4,400	0	4,400	0
Public Service Company of New Hampshire | Other
Summary of Accounting Policies [Line Items]
Other Income, Net	$ 0	$ 0	$ 0	$ 100