REGULATORY ACCOUNTING - Components of Regulatory Liabilities (Details) - USD ($) $ in Thousands	Dec. 31, 2016	Dec. 31, 2015
Regulatory Liabilities [Line Items]
Total Regulatory Liabilities	$ 849,100	$ 621,400
Less: Current Portion	146,787	107,759
Total Long-Term Regulatory Liabilities	702,255	513,595
Cost of Removal [Member]
Regulatory Liabilities [Line Items]
Total Regulatory Liabilities	459,700	437,100
Regulatory Tracker Mechanisms [Member]
Regulatory Liabilities [Line Items]
Total Regulatory Liabilities	145,300	99,700
Benefit Costs [Member]
Regulatory Liabilities [Line Items]
Total Regulatory Liabilities	136,200	0
AFUDC - Transmission [Member]
Regulatory Liabilities [Line Items]
Total Regulatory Liabilities	65,800	66,100
Other Regulatory Liabilities [Member]
Regulatory Liabilities [Line Items]
Total Regulatory Liabilities	42,100	18,500
The Connecticut Light And Power Company [Member]
Regulatory Liabilities [Line Items]
Total Regulatory Liabilities	147,200	136,000
Less: Current Portion	47,055	61,155
Total Long-Term Regulatory Liabilities	100,138	74,830
The Connecticut Light And Power Company [Member] | Cost of Removal [Member]
Regulatory Liabilities [Line Items]
Total Regulatory Liabilities	38,800	24,100
The Connecticut Light And Power Company [Member] | Regulatory Tracker Mechanisms [Member]
Regulatory Liabilities [Line Items]
Total Regulatory Liabilities	37,200	56,200
The Connecticut Light And Power Company [Member] | Benefit Costs [Member]
Regulatory Liabilities [Line Items]
Total Regulatory Liabilities	0	0
The Connecticut Light And Power Company [Member] | AFUDC - Transmission [Member]
Regulatory Liabilities [Line Items]
Total Regulatory Liabilities	50,200	51,500
The Connecticut Light And Power Company [Member] | Other Regulatory Liabilities [Member]
Regulatory Liabilities [Line Items]
Total Regulatory Liabilities	21,000	4,200
NSTAR Electric Company [Member]
Regulatory Liabilities [Line Items]
Total Regulatory Liabilities	455,500	267,700
Less: Current Portion	63,653	3,281
Total Long-Term Regulatory Liabilities	391,823	264,352
NSTAR Electric Company [Member] | Cost of Removal [Member]
Regulatory Liabilities [Line Items]
Total Regulatory Liabilities	271,600	257,400
NSTAR Electric Company [Member] | Regulatory Tracker Mechanisms [Member]
Regulatory Liabilities [Line Items]
Total Regulatory Liabilities	63,700	3,300
NSTAR Electric Company [Member] | Benefit Costs [Member]
Regulatory Liabilities [Line Items]
Total Regulatory Liabilities	113,100	0
NSTAR Electric Company [Member] | AFUDC - Transmission [Member]
Regulatory Liabilities [Line Items]
Total Regulatory Liabilities	6,900	5,700
NSTAR Electric Company [Member] | Other Regulatory Liabilities [Member]
Regulatory Liabilities [Line Items]
Total Regulatory Liabilities	200	1,300
Public Service Company Of New Hampshire [Member]
Regulatory Liabilities [Line Items]
Total Regulatory Liabilities	57,500	54,800
Less: Current Portion	12,659	6,898
Total Long-Term Regulatory Liabilities	44,779	47,851
Public Service Company Of New Hampshire [Member] | Cost of Removal [Member]
Regulatory Liabilities [Line Items]
Total Regulatory Liabilities	44,100	47,200
Public Service Company Of New Hampshire [Member] | Regulatory Tracker Mechanisms [Member]
Regulatory Liabilities [Line Items]
Total Regulatory Liabilities	10,700	3,400
Public Service Company Of New Hampshire [Member] | Benefit Costs [Member]
Regulatory Liabilities [Line Items]
Total Regulatory Liabilities	0	0
Public Service Company Of New Hampshire [Member] | AFUDC - Transmission [Member]
Regulatory Liabilities [Line Items]
Total Regulatory Liabilities	0	0
Public Service Company Of New Hampshire [Member] | Other Regulatory Liabilities [Member]
Regulatory Liabilities [Line Items]
Total Regulatory Liabilities	2,700	4,200
Western Massachusetts Electric Company [Member]
Regulatory Liabilities [Line Items]
Total Regulatory Liabilities	32,100	24,700
Less: Current Portion	14,888	13,122
Total Long-Term Regulatory Liabilities	17,227	11,597
Western Massachusetts Electric Company [Member] | Cost of Removal [Member]
Regulatory Liabilities [Line Items]
Total Regulatory Liabilities	8,600	2,800
Western Massachusetts Electric Company [Member] | Regulatory Tracker Mechanisms [Member]
Regulatory Liabilities [Line Items]
Total Regulatory Liabilities	14,700	12,900
Western Massachusetts Electric Company [Member] | Benefit Costs [Member]
Regulatory Liabilities [Line Items]
Total Regulatory Liabilities	0
Western Massachusetts Electric Company [Member] | AFUDC - Transmission [Member]
Regulatory Liabilities [Line Items]
Total Regulatory Liabilities	8,700	8,900
Western Massachusetts Electric Company [Member] | Other Regulatory Liabilities [Member]
Regulatory Liabilities [Line Items]
Total Regulatory Liabilities	$ 100	$ 100