REGULATORY ACCOUNTING - Components of Regulatory Assets (Details) - USD ($) $ in Thousands	Mar. 31, 2018	Dec. 31, 2017
Regulatory Assets [Line Items]
Total Regulatory Assets	$ 5,338,700	$ 5,239,300
Less: Current Portion	683,211	741,868
Total Long-Term Regulatory Assets	4,655,521	4,497,447
Benefit Costs
Regulatory Assets [Line Items]
Total Regulatory Assets	2,033,500	2,068,800
Income Taxes, Net
Regulatory Assets [Line Items]
Total Regulatory Assets	727,100	768,900
Deferred Costs from Generation Asset Sale
Regulatory Assets [Line Items]
Total Regulatory Assets	539,600	516,100
Storm Restoration Costs
Regulatory Assets [Line Items]
Total Regulatory Assets	544,400	404,800
Regulatory Tracker Mechanisms
Regulatory Assets [Line Items]
Total Regulatory Assets	461,100	509,900
Derivative Liabilities
Regulatory Assets [Line Items]
Total Regulatory Assets	388,000	367,200
Goodwill-related
Regulatory Assets [Line Items]
Total Regulatory Assets	361,000	365,200
Asset Retirement Obligations
Regulatory Assets [Line Items]
Total Regulatory Assets	88,900	101,000
Other Regulatory Assets
Regulatory Assets [Line Items]
Total Regulatory Assets	195,100	137,400
The Connecticut Light And Power Company
Regulatory Assets [Line Items]
Total Regulatory Assets	1,729,300	1,645,200
Less: Current Portion	229,735	200,281
Total Long-Term Regulatory Assets	1,499,588	1,444,935
The Connecticut Light And Power Company | Benefit Costs
Regulatory Assets [Line Items]
Total Regulatory Assets	460,600	469,200
The Connecticut Light And Power Company | Income Taxes, Net
Regulatory Assets [Line Items]
Total Regulatory Assets	451,800	453,800
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	254,400	216,700
The Connecticut Light And Power Company | Regulatory Tracker Mechanisms
Regulatory Assets [Line Items]
Total Regulatory Assets	118,000	85,300
The Connecticut Light And Power Company | Derivative Liabilities
Regulatory Assets [Line Items]
Total Regulatory Assets	386,500	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	30,800	30,300
The Connecticut Light And Power Company | Other Regulatory Assets
Regulatory Assets [Line Items]
Total Regulatory Assets	27,200	27,600
NSTAR Electric Company
Regulatory Assets [Line Items]
Total Regulatory Assets	1,564,300	1,524,500
Less: Current Portion	276,962	333,882
Total Long-Term Regulatory Assets	1,287,282	1,190,575
NSTAR Electric Company | Benefit Costs
Regulatory Assets [Line Items]
Total Regulatory Assets	549,900	560,700
NSTAR Electric Company | Income Taxes, Net
Regulatory Assets [Line Items]
Total Regulatory Assets	115,600	113,200
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	246,400	146,600
NSTAR Electric Company | Regulatory Tracker Mechanisms
Regulatory Assets [Line Items]
Total Regulatory Assets	221,400	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	310,000	313,600
NSTAR Electric Company | Asset Retirement Obligations
Regulatory Assets [Line Items]
Total Regulatory Assets	39,800	39,000
NSTAR Electric Company | Other Regulatory Assets
Regulatory Assets [Line Items]
Total Regulatory Assets	81,200	78,400
Public Service Company Of New Hampshire
Regulatory Assets [Line Items]
Total Regulatory Assets	938,700	940,800
Less: Current Portion	122,481	130,134
Total Long-Term Regulatory Assets	816,194	810,677
Public Service Company Of New Hampshire | Benefit Costs
Regulatory Assets [Line Items]
Total Regulatory Assets	208,600	212,300
Public Service Company Of New Hampshire | Income Taxes, Net
Regulatory Assets [Line Items]
Total Regulatory Assets	20,500	21,700
Public Service Company Of New Hampshire | Deferred Costs from Generation Asset Sale
Regulatory Assets [Line Items]
Total Regulatory Assets	539,600	516,100
Public Service Company Of New Hampshire | Storm Restoration Costs
Regulatory Assets [Line Items]
Total Regulatory Assets	43,600	41,500
Public Service Company Of New Hampshire | Regulatory Tracker Mechanisms
Regulatory Assets [Line Items]
Total Regulatory Assets	107,700	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	3,400	17,000
Public Service Company Of New Hampshire | Other Regulatory Assets
Regulatory Assets [Line Items]
Total Regulatory Assets	$ 15,300	$ 15,800