REGULATORY ACCOUNTING - Components of Regulatory Assets (Details) - USD ($) $ in Thousands	Sep. 30, 2018	Dec. 31, 2017
Regulatory Assets [Line Items]
Total Regulatory Assets	$ 5,153,300	$ 5,239,300
Less: Current Portion	436,704	741,868
Total Long-Term Regulatory Assets	4,716,631	4,497,447
Benefit Costs
Regulatory Assets [Line Items]
Total Regulatory Assets	1,988,400	2,068,800
Income Taxes, Net
Regulatory Assets [Line Items]
Total Regulatory Assets	720,700	768,900
Securitized Stranded Costs
Regulatory Assets [Line Items]
Total Regulatory Assets	618,600	0
Deferred Costs from Generation Asset Sale
Regulatory Assets [Line Items]
Total Regulatory Assets	0	516,100
Storm Restoration Costs
Regulatory Assets [Line Items]
Total Regulatory Assets	598,500	404,800
Regulatory Tracker Mechanisms
Regulatory Assets [Line Items]
Total Regulatory Assets	257,800	509,900
Derivative Liabilities
Regulatory Assets [Line Items]
Total Regulatory Assets	360,000	367,200
Goodwill-related
Regulatory Assets [Line Items]
Total Regulatory Assets	352,600	365,200
Asset Retirement Obligations
Regulatory Assets [Line Items]
Total Regulatory Assets	87,500	101,000
Other Regulatory Assets
Regulatory Assets [Line Items]
Total Regulatory Assets	169,200	137,400
The Connecticut Light and Power Company
Regulatory Assets [Line Items]
Total Regulatory Assets	1,673,800	1,645,200
Less: Current Portion	128,793	200,281
Total Long-Term Regulatory Assets	1,545,012	1,444,935
The Connecticut Light and Power Company | Benefit Costs
Regulatory Assets [Line Items]
Total Regulatory Assets	447,400	469,200
The Connecticut Light and Power Company | Income Taxes, Net
Regulatory Assets [Line Items]
Total Regulatory Assets	448,100	453,800
The Connecticut Light and Power Company | Securitized Stranded Costs
Regulatory Assets [Line Items]
Total Regulatory Assets	0	0
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 | Storm Restoration Costs
Regulatory Assets [Line Items]
Total Regulatory Assets	319,700	216,700
The Connecticut Light and Power Company | Regulatory Tracker Mechanisms
Regulatory Assets [Line Items]
Total Regulatory Assets	39,500	85,300
The Connecticut Light and Power Company | Derivative Liabilities
Regulatory Assets [Line Items]
Total Regulatory Assets	359,900	362,300
The Connecticut Light and Power Company | Goodwill-related
Regulatory Assets [Line Items]
Total Regulatory Assets	0	0
The Connecticut Light and Power Company | Asset Retirement Obligations
Regulatory Assets [Line Items]
Total Regulatory Assets	31,800	30,300
The Connecticut Light and Power Company | Other Regulatory Assets
Regulatory Assets [Line Items]
Total Regulatory Assets	27,400	27,600
NSTAR Electric Company
Regulatory Assets [Line Items]
Total Regulatory Assets	1,420,300	1,524,500
Less: Current Portion	193,541	333,882
Total Long-Term Regulatory Assets	1,226,811	1,190,575
NSTAR Electric Company | Benefit Costs
Regulatory Assets [Line Items]
Total Regulatory Assets	541,200	560,700
NSTAR Electric Company | Income Taxes, Net
Regulatory Assets [Line Items]
Total Regulatory Assets	112,900	113,200
NSTAR Electric Company | Securitized Stranded Costs
Regulatory Assets [Line Items]
Total Regulatory Assets	0	0
NSTAR Electric Company | Deferred Costs from Generation Asset Sale
Regulatory Assets [Line Items]
Total Regulatory Assets	0	0
NSTAR Electric Company | Storm Restoration Costs
Regulatory Assets [Line Items]
Total Regulatory Assets	222,100	146,600
NSTAR Electric Company | Regulatory Tracker Mechanisms
Regulatory Assets [Line Items]
Total Regulatory Assets	128,400	273,000
NSTAR Electric Company | Derivative Liabilities
Regulatory Assets [Line Items]
Total Regulatory Assets	0	0
NSTAR Electric Company | Goodwill-related
Regulatory Assets [Line Items]
Total Regulatory Assets	302,700	313,600
NSTAR Electric Company | Asset Retirement Obligations
Regulatory Assets [Line Items]
Total Regulatory Assets	41,300	39,000
NSTAR Electric Company | Other Regulatory Assets
Regulatory Assets [Line Items]
Total Regulatory Assets	71,700	78,400
Public Service Company of New Hampshire
Regulatory Assets [Line Items]
Total Regulatory Assets	953,500	940,800
Less: Current Portion	61,379	130,134
Total Long-Term Regulatory Assets	892,075	810,677
Public Service Company of New Hampshire | Benefit Costs
Regulatory Assets [Line Items]
Total Regulatory Assets	172,000	212,300
Public Service Company of New Hampshire | Income Taxes, Net
Regulatory Assets [Line Items]
Total Regulatory Assets	15,600	21,700
Public Service Company of New Hampshire | Securitized Stranded Costs
Regulatory Assets [Line Items]
Total Regulatory Assets	618,600	0
Public Service Company of New Hampshire | Deferred Costs from Generation Asset Sale
Regulatory Assets [Line Items]
Total Regulatory Assets	0	516,100
Public Service Company of New Hampshire | Storm Restoration Costs
Regulatory Assets [Line Items]
Total Regulatory Assets	56,700	41,500
Public Service Company of New Hampshire | Regulatory Tracker Mechanisms
Regulatory Assets [Line Items]
Total Regulatory Assets	74,900	116,400
Public Service Company of New Hampshire | Derivative Liabilities
Regulatory Assets [Line Items]
Total Regulatory Assets	0	0
Public Service Company of New Hampshire | Goodwill-related
Regulatory Assets [Line Items]
Total Regulatory Assets	0	0
Public Service Company of New Hampshire | Asset Retirement Obligations
Regulatory Assets [Line Items]
Total Regulatory Assets	3,300	17,000
Public Service Company of New Hampshire | Other Regulatory Assets
Regulatory Assets [Line Items]
Total Regulatory Assets	$ 12,400	$ 15,800