REGULATORY ACCOUNTING - Components of Regulatory Assets (Details) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Regulatory Assets [Line Items]
Total Regulatory Assets	$ 5,239,300	$ 4,526,300
Less: Current Portion	741,868	887,625
Total Long-Term Regulatory Assets	4,497,447	3,638,688
Benefit Costs
Regulatory Assets [Line Items]
Total Regulatory Assets	2,068,800	1,817,800
Deferred Costs from Generation Asset Sale
Regulatory Assets [Line Items]
Total Regulatory Assets	516,100	0
Derivative Liabilities
Regulatory Assets [Line Items]
Total Regulatory Assets	367,200	423,300
Income Taxes, Net
Regulatory Assets [Line Items]
Total Regulatory Assets	768,900	644,500
Storm Restoration Costs
Regulatory Assets [Line Items]
Total Regulatory Assets	404,800	385,300
Goodwill-related
Regulatory Assets [Line Items]
Total Regulatory Assets	365,200	464,400
Regulatory Tracker Mechanisms
Regulatory Assets [Line Items]
Total Regulatory Assets	509,900	576,600
Asset Retirement Obligations
Regulatory Assets [Line Items]
Total Regulatory Assets	101,000	99,300
Other Regulatory Assets
Regulatory Assets [Line Items]
Total Regulatory Assets	137,400	115,100
The Connecticut Light And Power Company
Regulatory Assets [Line Items]
Total Regulatory Assets	1,645,200	1,727,100
Less: Current Portion	200,281	335,526
Total Long-Term Regulatory Assets	1,444,935	1,391,564
The Connecticut Light And Power Company | Benefit Costs
Regulatory Assets [Line Items]
Total Regulatory Assets	469,200	429,300
The Connecticut Light And Power Company | Deferred Costs from Generation Asset Sale
Regulatory Assets [Line Items]
Total Regulatory Assets	0	0
The Connecticut Light And Power Company | Derivative Liabilities
Regulatory Assets [Line Items]
Total Regulatory Assets	362,300	420,500
The Connecticut Light And Power Company | Income Taxes, Net
Regulatory Assets [Line Items]
Total Regulatory Assets	453,800	437,000
The Connecticut Light And Power Company | Storm Restoration Costs
Regulatory Assets [Line Items]
Total Regulatory Assets	216,700	239,800
The Connecticut Light And Power Company | Goodwill-related
Regulatory Assets [Line Items]
Total Regulatory Assets	0	0
The Connecticut Light And Power Company | Regulatory Tracker Mechanisms
Regulatory Assets [Line Items]
Total Regulatory Assets	85,300	123,900
The Connecticut Light And Power Company | Asset Retirement Obligations
Regulatory Assets [Line Items]
Total Regulatory Assets	30,300	33,200
The Connecticut Light And Power Company | Other Regulatory Assets
Regulatory Assets [Line Items]
Total Regulatory Assets	27,600	43,400
NSTAR Electric Company
Regulatory Assets [Line Items]
Total Regulatory Assets	1,524,500	1,538,500
Less: Current Portion	333,882	353,522
Total Long-Term Regulatory Assets	1,190,575	1,185,037
NSTAR Electric Company | Benefit Costs
Regulatory Assets [Line Items]
Total Regulatory Assets	560,700	525,300
NSTAR Electric Company | Deferred Costs from Generation Asset Sale
Regulatory Assets [Line Items]
Total Regulatory Assets	0	0
NSTAR Electric Company | Derivative Liabilities
Regulatory Assets [Line Items]
Total Regulatory Assets	0	2,800
NSTAR Electric Company | Income Taxes, Net
Regulatory Assets [Line Items]
Total Regulatory Assets	113,200	120,500
NSTAR Electric Company | Storm Restoration Costs
Regulatory Assets [Line Items]
Total Regulatory Assets	146,600	128,400
NSTAR Electric Company | Goodwill-related
Regulatory Assets [Line Items]
Total Regulatory Assets	313,600	398,700
NSTAR Electric Company | Regulatory Tracker Mechanisms
Regulatory Assets [Line Items]
Total Regulatory Assets	273,000	304,000
NSTAR Electric Company | Asset Retirement Obligations
Regulatory Assets [Line Items]
Total Regulatory Assets	39,000	36,100
NSTAR Electric Company | Other Regulatory Assets
Regulatory Assets [Line Items]
Total Regulatory Assets	78,400	22,700
Public Service Company Of New Hampshire
Regulatory Assets [Line Items]
Total Regulatory Assets	940,800	362,700
Less: Current Portion	130,134	117,240
Total Long-Term Regulatory Assets	810,677	245,525
Public Service Company Of New Hampshire | Benefit Costs
Regulatory Assets [Line Items]
Total Regulatory Assets	212,300	184,200
Public Service Company Of New Hampshire | Deferred Costs from Generation Asset Sale
Regulatory Assets [Line Items]
Total Regulatory Assets	516,100	0
Public Service Company Of New Hampshire | Derivative Liabilities
Regulatory Assets [Line Items]
Total Regulatory Assets	0	0
Public Service Company Of New Hampshire | Income Taxes, Net
Regulatory Assets [Line Items]
Total Regulatory Assets	21,700	24,200
Public Service Company Of New Hampshire | Storm Restoration Costs
Regulatory Assets [Line Items]
Total Regulatory Assets	41,500	17,100
Public Service Company Of New Hampshire | Goodwill-related
Regulatory Assets [Line Items]
Total Regulatory Assets	0	0
Public Service Company Of New Hampshire | Regulatory Tracker Mechanisms
Regulatory Assets [Line Items]
Total Regulatory Assets	116,400	104,500
Public Service Company Of New Hampshire | Asset Retirement Obligations
Regulatory Assets [Line Items]
Total Regulatory Assets	17,000	16,200
Public Service Company Of New Hampshire | Other Regulatory Assets
Regulatory Assets [Line Items]
Total Regulatory Assets	$ 15,800	$ 16,500