SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES - Other Income (Details) - USD ($) $ in Thousands	3 Months Ended	12 Months Ended
	Mar. 31, 2019	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Summary of Accounting Policies [Line Items]
Other Income, Net		$ 132,777	$ 128,366	$ 107,913
Other-than-temporary impairment			32,900
Unrealized gain associated with investment		42,200	3,800	27,400
Pension, SERP and PBOP Non-Service Income Components
Summary of Accounting Policies [Line Items]
Other Income, Net		31,300	60,800	29,900
AFUDC Equity
Summary of Accounting Policies [Line Items]
Other Income, Net		45,000	44,000	34,400
Equity in Earnings of Unconsolidated Affiliates
Summary of Accounting Policies [Line Items]
Other Income, Net		42,200	3,800	27,400
Unrealized gain associated with investment		20,400	17,600	9,700
Investment Income/(Loss)
Summary of Accounting Policies [Line Items]
Other Income, Net		800	(4,000)	7,500
Interest Income
Summary of Accounting Policies [Line Items]
Other Income, Net		12,800	18,100	8,300
Gains on Sales of Property
Summary of Accounting Policies [Line Items]
Other Income, Net		300	5,100	0
Other
Summary of Accounting Policies [Line Items]
Other Income, Net		400	600	400
CL&P
Summary of Accounting Policies [Line Items]
Other Income, Net		17,531	22,663	22,991
CL&P | Pension, SERP and PBOP Non-Service Income Components
Summary of Accounting Policies [Line Items]
Other Income, Net		500	9,500	1,800
CL&P | AFUDC Equity
Summary of Accounting Policies [Line Items]
Other Income, Net		13,200	12,200	12,100
CL&P | Equity in Earnings of Unconsolidated Affiliates
Summary of Accounting Policies [Line Items]
Other Income, Net		100	100	0
CL&P | Investment Income/(Loss)
Summary of Accounting Policies [Line Items]
Other Income, Net		2,300	(3,000)	4,500
CL&P | Interest Income
Summary of Accounting Policies [Line Items]
Other Income, Net		1,500	3,700	4,600
CL&P | Gains on Sales of Property
Summary of Accounting Policies [Line Items]
Other Income, Net		0	0	0
CL&P | Other
Summary of Accounting Policies [Line Items]
Other Income, Net		(100)	200	0
NSTAR Electric
Summary of Accounting Policies [Line Items]
Other Income, Net		44,577	53,066	34,100
NSTAR Electric | Pension, SERP and PBOP Non-Service Income Components
Summary of Accounting Policies [Line Items]
Other Income, Net		23,500	36,000	19,200
NSTAR Electric | AFUDC Equity
Summary of Accounting Policies [Line Items]
Other Income, Net		19,800	15,600	10,200
NSTAR Electric | Equity in Earnings of Unconsolidated Affiliates
Summary of Accounting Policies [Line Items]
Other Income, Net		700	700	300
NSTAR Electric | Investment Income/(Loss)
Summary of Accounting Policies [Line Items]
Other Income, Net		(400)	(500)	2,600
NSTAR Electric | Interest Income
Summary of Accounting Policies [Line Items]
Other Income, Net		700	800	1,800
NSTAR Electric | Gains on Sales of Property
Summary of Accounting Policies [Line Items]
Other Income, Net		100	500	0
NSTAR Electric | Other
Summary of Accounting Policies [Line Items]
Other Income, Net		200	0	0
PSNH
Summary of Accounting Policies [Line Items]
Other Income, Net		19,222	27,672	9,805
PSNH | Pension, SERP and PBOP Non-Service Income Components
Summary of Accounting Policies [Line Items]
Other Income, Net		4,900	9,900	5,900
PSNH | AFUDC Equity
Summary of Accounting Policies [Line Items]
Other Income, Net		3,400	0	0
PSNH | Equity in Earnings of Unconsolidated Affiliates
Summary of Accounting Policies [Line Items]
Other Income, Net		0	0	0
PSNH | Investment Income/(Loss)
Summary of Accounting Policies [Line Items]
Other Income, Net		300	(800)	1,600
PSNH | Interest Income
Summary of Accounting Policies [Line Items]
Other Income, Net		10,500	14,100	2,200
PSNH | Gains on Sales of Property
Summary of Accounting Policies [Line Items]
Other Income, Net		0	4,400	0
PSNH | Other
Summary of Accounting Policies [Line Items]
Other Income, Net		100	100	$ 100
Deferred Storm Restoration Costs | PSNH
Summary of Accounting Policies [Line Items]
Other Income, Net	$ 5,200	$ 6,300	$ 8,700