REGULATORY ACCOUNTING - Components of Regulatory Assets (Details) - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017
Regulatory Assets [Line Items]
Total Regulatory Assets	$ 5,145,900	$ 5,239,300
Less: Current Portion	514,779	741,868
Total Long-Term Regulatory Assets	4,631,137	4,497,447
Benefit Costs
Regulatory Assets [Line Items]
Total Regulatory Assets	1,914,800	2,068,800
Income Taxes, Net
Regulatory Assets [Line Items]
Total Regulatory Assets	728,600	768,900
Securitized Stranded Costs
Regulatory Assets [Line Items]
Total Regulatory Assets	608,400	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	576,000	404,800
Regulatory Tracker Mechanisms
Regulatory Assets [Line Items]
Total Regulatory Assets	316,000	509,900
Derivative Liabilities
Regulatory Assets [Line Items]
Total Regulatory Assets	356,500	367,200
Goodwill-related
Regulatory Assets [Line Items]
Total Regulatory Assets	348,400	365,200
Asset Retirement Obligations
Regulatory Assets [Line Items]
Total Regulatory Assets	89,200	101,000
Other Regulatory Assets
Regulatory Assets [Line Items]
Total Regulatory Assets	208,000	137,400
The Connecticut Light And Power Company
Regulatory Assets [Line Items]
Total Regulatory Assets	1,630,700	1,645,200
Less: Current Portion	125,155	200,281
Total Long-Term Regulatory Assets	1,505,488	1,444,935
The Connecticut Light And Power Company | Benefit Costs
Regulatory Assets [Line Items]
Total Regulatory Assets	424,700	469,200
The Connecticut Light And Power Company | Income Taxes, Net
Regulatory Assets [Line Items]
Total Regulatory Assets	454,400	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	302,600	216,700
The Connecticut Light And Power Company | Regulatory Tracker Mechanisms
Regulatory Assets [Line Items]
Total Regulatory Assets	33,200	85,300
The Connecticut Light And Power Company | Derivative Liabilities
Regulatory Assets [Line Items]
Total Regulatory Assets	356,500	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	32,300	30,300
The Connecticut Light And Power Company | Other Regulatory Assets
Regulatory Assets [Line Items]
Total Regulatory Assets	27,000	27,600
NSTAR Electric Company
Regulatory Assets [Line Items]
Total Regulatory Assets	1,438,200	1,524,500
Less: Current Portion	241,747	333,882
Total Long-Term Regulatory Assets	1,196,512	1,190,575
NSTAR Electric Company | Benefit Costs
Regulatory Assets [Line Items]
Total Regulatory Assets	544,400	560,700
NSTAR Electric Company | Income Taxes, Net
Regulatory Assets [Line Items]
Total Regulatory Assets	105,900	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	212,900	146,600
NSTAR Electric Company | Regulatory Tracker Mechanisms
Regulatory Assets [Line Items]
Total Regulatory Assets	169,100	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	299,100	313,600
NSTAR Electric Company | Asset Retirement Obligations
Regulatory Assets [Line Items]
Total Regulatory Assets	42,200	39,000
NSTAR Electric Company | Other Regulatory Assets
Regulatory Assets [Line Items]
Total Regulatory Assets	64,600	78,400
Public Service Company Of New Hampshire
Regulatory Assets [Line Items]
Total Regulatory Assets	929,500	940,800
Less: Current Portion	67,228	130,134
Total Long-Term Regulatory Assets	862,288	810,677
Public Service Company Of New Hampshire | Benefit Costs
Regulatory Assets [Line Items]
Total Regulatory Assets	169,600	212,300
Public Service Company Of New Hampshire | Income Taxes, Net
Regulatory Assets [Line Items]
Total Regulatory Assets	8,300	21,700
Public Service Company Of New Hampshire | Securitized Stranded Costs
Regulatory Assets [Line Items]
Total Regulatory Assets		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	60,500	41,500
Public Service Company Of New Hampshire | Regulatory Tracker Mechanisms
Regulatory Assets [Line Items]
Total Regulatory Assets	67,300	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	$ 12,100	$ 15,800