REGULATORY ACCOUNTING - Components of Regulatory Liabilities (Details) - USD ($) $ in Thousands	Jun. 30, 2017	Dec. 31, 2016
Regulatory Liabilities [Line Items]
Total Regulatory Liabilities	$ 882,600	$ 849,100
Less: Current Portion	185,930	146,787
Total Long-Term Regulatory Liabilities	696,740	702,255
Cost of Removal [Member]
Regulatory Liabilities [Line Items]
Total Regulatory Liabilities	474,400	459,700
Benefit Costs [Member]
Regulatory Liabilities [Line Items]
Total Regulatory Liabilities	129,500	136,200
Regulatory Tracker Mechanisms [Member]
Regulatory Liabilities [Line Items]
Total Regulatory Liabilities	176,300	145,300
AFUDC - Transmission [Member]
Regulatory Liabilities [Line Items]
Total Regulatory Liabilities	65,600	65,800
Other Regulatory Liabilities [Member]
Regulatory Liabilities [Line Items]
Total Regulatory Liabilities	36,800	42,100
The Connecticut Light And Power Company [Member]
Regulatory Liabilities [Line Items]
Total Regulatory Liabilities	164,100	147,200
Less: Current Portion	62,195	47,055
Total Long-Term Regulatory Liabilities	101,863	100,138
The Connecticut Light And Power Company [Member] | Cost of Removal [Member]
Regulatory Liabilities [Line Items]
Total Regulatory Liabilities	43,400	38,800
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] | Regulatory Tracker Mechanisms [Member]
Regulatory Liabilities [Line Items]
Total Regulatory Liabilities	47,500	37,200
The Connecticut Light And Power Company [Member] | AFUDC - Transmission [Member]
Regulatory Liabilities [Line Items]
Total Regulatory Liabilities	49,500	50,200
The Connecticut Light And Power Company [Member] | Other Regulatory Liabilities [Member]
Regulatory Liabilities [Line Items]
Total Regulatory Liabilities	23,700	21,000
NSTAR Electric Company [Member]
Regulatory Liabilities [Line Items]
Total Regulatory Liabilities	442,900	455,500
Less: Current Portion	48,350	63,653
Total Long-Term Regulatory Liabilities	394,527	391,823
NSTAR Electric Company [Member] | Cost of Removal [Member]
Regulatory Liabilities [Line Items]
Total Regulatory Liabilities	277,400	271,600
NSTAR Electric Company [Member] | Benefit Costs [Member]
Regulatory Liabilities [Line Items]
Total Regulatory Liabilities	109,400	113,100
NSTAR Electric Company [Member] | Regulatory Tracker Mechanisms [Member]
Regulatory Liabilities [Line Items]
Total Regulatory Liabilities	48,400	63,700
NSTAR Electric Company [Member] | AFUDC - Transmission [Member]
Regulatory Liabilities [Line Items]
Total Regulatory Liabilities	7,400	6,900
NSTAR Electric Company [Member] | Other Regulatory Liabilities [Member]
Regulatory Liabilities [Line Items]
Total Regulatory Liabilities	300	200
Public Service Company Of New Hampshire [Member]
Regulatory Liabilities [Line Items]
Total Regulatory Liabilities	57,500	57,500
Less: Current Portion	13,696	12,659
Total Long-Term Regulatory Liabilities	43,845	44,779
Public Service Company Of New Hampshire [Member] | Cost of Removal [Member]
Regulatory Liabilities [Line Items]
Total Regulatory Liabilities	42,800	44,100
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] | Regulatory Tracker Mechanisms [Member]
Regulatory Liabilities [Line Items]
Total Regulatory Liabilities	11,700	10,700
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	3,000	2,700
Western Massachusetts Electric Company [Member]
Regulatory Liabilities [Line Items]
Total Regulatory Liabilities	32,000	32,100
Less: Current Portion	11,062	14,888
Total Long-Term Regulatory Liabilities	20,887	17,227
Western Massachusetts Electric Company [Member] | Cost of Removal [Member]
Regulatory Liabilities [Line Items]
Total Regulatory Liabilities	12,400	8,600
Western Massachusetts Electric Company [Member] | Benefit Costs [Member]
Regulatory Liabilities [Line Items]
Total Regulatory Liabilities	0	0
Western Massachusetts Electric Company [Member] | Regulatory Tracker Mechanisms [Member]
Regulatory Liabilities [Line Items]
Total Regulatory Liabilities	10,900	14,700
Western Massachusetts Electric Company [Member] | AFUDC - Transmission [Member]
Regulatory Liabilities [Line Items]
Total Regulatory Liabilities	8,700	8,700
Western Massachusetts Electric Company [Member] | Other Regulatory Liabilities [Member]
Regulatory Liabilities [Line Items]
Total Regulatory Liabilities	$ 0	$ 100