REGULATORY ACCOUNTING - Components of Regulatory Liabilities (Details) - USD ($) $ in Thousands	Sep. 30, 2017	Dec. 31, 2016
Regulatory Liabilities [Line Items]
Total Regulatory Liabilities	$ 870,400	$ 849,100
Less: Current Portion	170,215	146,787
Total Long-Term Regulatory Liabilities	700,207	702,255
Cost of Removal
Regulatory Liabilities [Line Items]
Total Regulatory Liabilities	470,300	459,700
Benefit Costs
Regulatory Liabilities [Line Items]
Total Regulatory Liabilities	125,500	136,200
Regulatory Tracker Mechanisms
Regulatory Liabilities [Line Items]
Total Regulatory Liabilities	175,800	145,300
AFUDC - Transmission
Regulatory Liabilities [Line Items]
Total Regulatory Liabilities	65,400	65,800
Other Regulatory Liabilities
Regulatory Liabilities [Line Items]
Total Regulatory Liabilities	33,400	42,100
The Connecticut Light And Power Company
Regulatory Liabilities [Line Items]
Total Regulatory Liabilities	168,100	147,200
Less: Current Portion	69,296	47,055
Total Long-Term Regulatory Liabilities	98,777	100,138
The Connecticut Light And Power Company | Cost of Removal
Regulatory Liabilities [Line Items]
Total Regulatory Liabilities	40,500	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	57,100	37,200
The Connecticut Light And Power Company | AFUDC - Transmission
Regulatory Liabilities [Line Items]
Total Regulatory Liabilities	49,200	50,200
The Connecticut Light And Power Company | Other Regulatory Liabilities
Regulatory Liabilities [Line Items]
Total Regulatory Liabilities	21,300	21,000
NSTAR Electric Company
Regulatory Liabilities [Line Items]
Total Regulatory Liabilities	458,400	455,500
Less: Current Portion	65,520	63,653
Total Long-Term Regulatory Liabilities	392,851	391,823
NSTAR Electric Company | Cost of Removal
Regulatory Liabilities [Line Items]
Total Regulatory Liabilities	278,800	271,600
NSTAR Electric Company | Benefit Costs
Regulatory Liabilities [Line Items]
Total Regulatory Liabilities	106,000	113,100
NSTAR Electric Company | Regulatory Tracker Mechanisms
Regulatory Liabilities [Line Items]
Total Regulatory Liabilities	65,500	63,700
NSTAR Electric Company | AFUDC - Transmission
Regulatory Liabilities [Line Items]
Total Regulatory Liabilities	7,700	6,900
NSTAR Electric Company | Other Regulatory Liabilities
Regulatory Liabilities [Line Items]
Total Regulatory Liabilities	400	200
Public Service Company Of New Hampshire
Regulatory Liabilities [Line Items]
Total Regulatory Liabilities	48,700	57,500
Less: Current Portion	7,923	12,659
Total Long-Term Regulatory Liabilities	40,822	44,779
Public Service Company Of New Hampshire | Cost of Removal
Regulatory Liabilities [Line Items]
Total Regulatory Liabilities	40,500	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	5,600	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	2,600	2,700
Western Massachusetts Electric Company
Regulatory Liabilities [Line Items]
Total Regulatory Liabilities	32,900	32,100
Less: Current Portion	10,236	14,888
Total Long-Term Regulatory Liabilities	22,726	17,227
Western Massachusetts Electric Company | Cost of Removal
Regulatory Liabilities [Line Items]
Total Regulatory Liabilities	11,700	8,600
Western Massachusetts Electric Company | Benefit Costs
Regulatory Liabilities [Line Items]
Total Regulatory Liabilities	0	0
Western Massachusetts Electric Company | Regulatory Tracker Mechanisms
Regulatory Liabilities [Line Items]
Total Regulatory Liabilities	12,700	14,700
Western Massachusetts Electric Company | AFUDC - Transmission
Regulatory Liabilities [Line Items]
Total Regulatory Liabilities	8,500	8,700
Western Massachusetts Electric Company | Other Regulatory Liabilities
Regulatory Liabilities [Line Items]
Total Regulatory Liabilities	$ 0	$ 100