REGULATORY ACCOUNTING - Components of Regulatory Liabilities (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017
Regulatory Liabilities [Line Items]
Total Regulatory Liabilities	$ 3,979,700	$ 3,765,400
Less: Current Portion	370,230	128,071
Total Long-Term Regulatory Liabilities	$ 3,609,475	3,637,273
Amount of regulated EDIT liabilities, percentage	85.00%
Excess ADIT due to tax Cuts and Jobs Act [Member]
Regulatory Liabilities [Line Items]
Total Regulatory Liabilities	$ 2,883,000	2,882,000
Cost of Removal
Regulatory Liabilities [Line Items]
Total Regulatory Liabilities	521,000	502,100
Benefit Costs
Regulatory Liabilities [Line Items]
Total Regulatory Liabilities	91,200	132,300
Regulatory Tracker Mechanisms
Regulatory Liabilities [Line Items]
Total Regulatory Liabilities	309,000	136,700
AFUDC - Transmission
Regulatory Liabilities [Line Items]
Total Regulatory Liabilities	70,700	67,100
Revenue Subject to Refund [Member]
Regulatory Liabilities [Line Items]
Total Regulatory Liabilities	24,600	0
Other Regulatory Liabilities
Regulatory Liabilities [Line Items]
Total Regulatory Liabilities	80,200	45,200
The Connecticut Light And Power Company
Regulatory Liabilities [Line Items]
Total Regulatory Liabilities	1,231,800	1,151,100
Less: Current Portion	109,614	38,967
Total Long-Term Regulatory Liabilities	1,122,157	1,112,136
The Connecticut Light And Power Company | Excess ADIT due to tax Cuts and Jobs Act [Member]
Regulatory Liabilities [Line Items]
Total Regulatory Liabilities	1,031,000	1,031,600
The Connecticut Light And Power Company | Cost of Removal
Regulatory Liabilities [Line Items]
Total Regulatory Liabilities	39,900	23,200
The Connecticut Light And Power Company | Benefit Costs
Regulatory Liabilities [Line Items]
Total Regulatory Liabilities	0	0
The Connecticut Light And Power Company | Regulatory Tracker Mechanisms
Regulatory Liabilities [Line Items]
Total Regulatory Liabilities	89,500	34,600
The Connecticut Light And Power Company | AFUDC - Transmission
Regulatory Liabilities [Line Items]
Total Regulatory Liabilities	47,400	48,800
The Connecticut Light And Power Company | Revenue Subject to Refund [Member]
Regulatory Liabilities [Line Items]
Total Regulatory Liabilities	0	0
The Connecticut Light And Power Company | Other Regulatory Liabilities
Regulatory Liabilities [Line Items]
Total Regulatory Liabilities	24,000	12,900
NSTAR Electric Company
Regulatory Liabilities [Line Items]
Total Regulatory Liabilities	1,703,900	1,594,100
Less: Current Portion	190,620	79,562
Total Long-Term Regulatory Liabilities	1,513,279	1,514,451
NSTAR Electric Company | Excess ADIT due to tax Cuts and Jobs Act [Member]
Regulatory Liabilities [Line Items]
Total Regulatory Liabilities	1,103,700	1,087,900
NSTAR Electric Company | Cost of Removal
Regulatory Liabilities [Line Items]
Total Regulatory Liabilities	307,100	293,800
NSTAR Electric Company | Benefit Costs
Regulatory Liabilities [Line Items]
Total Regulatory Liabilities	76,900	112,600
NSTAR Electric Company | Regulatory Tracker Mechanisms
Regulatory Liabilities [Line Items]
Total Regulatory Liabilities	163,700	77,800
NSTAR Electric Company | AFUDC - Transmission
Regulatory Liabilities [Line Items]
Total Regulatory Liabilities	23,300	18,300
NSTAR Electric Company | Revenue Subject to Refund [Member]
Regulatory Liabilities [Line Items]
Total Regulatory Liabilities	0	0
NSTAR Electric Company | Other Regulatory Liabilities
Regulatory Liabilities [Line Items]
Total Regulatory Liabilities	29,200	3,700
Public Service Company Of New Hampshire
Regulatory Liabilities [Line Items]
Total Regulatory Liabilities	483,600	450,700
Less: Current Portion	55,526	6,251
Total Long-Term Regulatory Liabilities	428,069	444,397
Public Service Company Of New Hampshire | Excess ADIT due to tax Cuts and Jobs Act [Member]
Regulatory Liabilities [Line Items]
Total Regulatory Liabilities	396,400	405,100
Public Service Company Of New Hampshire | Cost of Removal
Regulatory Liabilities [Line Items]
Total Regulatory Liabilities	22,100	37,900
Public Service Company Of New Hampshire | Benefit Costs
Regulatory Liabilities [Line Items]
Total Regulatory Liabilities	0	0
Public Service Company Of New Hampshire | Regulatory Tracker Mechanisms
Regulatory Liabilities [Line Items]
Total Regulatory Liabilities	48,300	5,000
Public Service Company Of New Hampshire | AFUDC - Transmission
Regulatory Liabilities [Line Items]
Total Regulatory Liabilities	0	0
Public Service Company Of New Hampshire | Revenue Subject to Refund [Member]
Regulatory Liabilities [Line Items]
Total Regulatory Liabilities	12,600	0
Public Service Company Of New Hampshire | Other Regulatory Liabilities
Regulatory Liabilities [Line Items]
Total Regulatory Liabilities	$ 4,200	$ 2,700
Minimum
Regulatory Liabilities [Line Items]
Estimated useful lives	35