REGULATORY ACCOUNTING - Components of Regulatory Liabilities (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016
Regulatory Liabilities [Line Items]
Total Regulatory Liabilities	$ 883,400	$ 849,100
Less: Current Portion	128,071	146,787
Total Long-Term Regulatory Liabilities	3,637,273	702,255
Deferred Income tax benefit related to tax cuts and jobs act	$ 2,900,000
Percentage of provisional regulated excess tax	85.00%
Cost of Removal
Regulatory Liabilities [Line Items]
Total Regulatory Liabilities	$ 502,100	459,700
Benefit Costs
Regulatory Liabilities [Line Items]
Total Regulatory Liabilities	132,300	136,200
Regulatory Tracker Mechanisms
Regulatory Liabilities [Line Items]
Total Regulatory Liabilities	136,700	145,300
AFUDC - Transmission
Regulatory Liabilities [Line Items]
Total Regulatory Liabilities	67,100	65,800
Other Regulatory Liabilities
Regulatory Liabilities [Line Items]
Total Regulatory Liabilities	45,200	42,100
Current Regulatory Liabilities
Regulatory Liabilities [Line Items]
Total Long-Term Regulatory Liabilities	755,300	702,300
The Connecticut Light And Power Company
Regulatory Liabilities [Line Items]
Total Regulatory Liabilities	119,500	147,200
Less: Current Portion	38,967	47,055
Total Long-Term Regulatory Liabilities	1,112,136	100,138
Deferred Income tax benefit related to tax cuts and jobs act	1,000,000
The Connecticut Light And Power Company | Cost of Removal
Regulatory Liabilities [Line Items]
Total Regulatory Liabilities	23,200	38,800
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	34,600	37,200
The Connecticut Light And Power Company | AFUDC - Transmission
Regulatory Liabilities [Line Items]
Total Regulatory Liabilities	48,800	50,200
The Connecticut Light And Power Company | Other Regulatory Liabilities
Regulatory Liabilities [Line Items]
Total Regulatory Liabilities	12,900	21,000
The Connecticut Light And Power Company | Non Current Regulatory Liabilities
Regulatory Liabilities [Line Items]
Total Long-Term Regulatory Liabilities	80,500	100,100
NSTAR Electric Company
Regulatory Liabilities [Line Items]
Total Regulatory Liabilities	506,200	487,600
Less: Current Portion	79,562	78,541
Total Long-Term Regulatory Liabilities	1,514,451	409,050
Deferred Income tax benefit related to tax cuts and jobs act	1,100,000
NSTAR Electric Company | Cost of Removal
Regulatory Liabilities [Line Items]
Total Regulatory Liabilities	293,800	280,200
NSTAR Electric Company | Benefit Costs
Regulatory Liabilities [Line Items]
Total Regulatory Liabilities	112,600	113,100
NSTAR Electric Company | Regulatory Tracker Mechanisms
Regulatory Liabilities [Line Items]
Total Regulatory Liabilities	77,800	78,400
NSTAR Electric Company | AFUDC - Transmission
Regulatory Liabilities [Line Items]
Total Regulatory Liabilities	18,300	15,600
NSTAR Electric Company | Other Regulatory Liabilities
Regulatory Liabilities [Line Items]
Total Regulatory Liabilities	3,700	300
NSTAR Electric Company | Non Current Regulatory Liabilities
Regulatory Liabilities [Line Items]
Total Long-Term Regulatory Liabilities	426,600	409,100
Public Service Company Of New Hampshire
Regulatory Liabilities [Line Items]
Total Regulatory Liabilities	45,600	57,500
Less: Current Portion	6,251	12,659
Total Long-Term Regulatory Liabilities	444,397	44,779
Deferred Income tax benefit related to tax cuts and jobs act	400,000
Public Service Company Of New Hampshire | Cost of Removal
Regulatory Liabilities [Line Items]
Total Regulatory Liabilities	37,900	44,100
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	5,000	10,700
Public Service Company Of New Hampshire | AFUDC - Transmission
Regulatory Liabilities [Line Items]
Total Regulatory Liabilities	0	0
Public Service Company Of New Hampshire | Other Regulatory Liabilities
Regulatory Liabilities [Line Items]
Total Regulatory Liabilities	2,700	2,700
Public Service Company Of New Hampshire | Non Current Regulatory Liabilities
Regulatory Liabilities [Line Items]
Total Long-Term Regulatory Liabilities	$ 39,300	$ 44,800
Minimum
Regulatory Liabilities [Line Items]
Estimated useful lives	20