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COVER PAGE - USD ($)
 12 Months Ended
Dec. 31, 2020
Jan. 31, 2021
Jun. 30, 2020
Entity Information [Line Items]      
Document Type 10-K    
Document Annual Report true    
Document Period End Date Dec. 31, 2020    
Current Fiscal Year End Date --12-31    
Document Transition Report false    
Entity Registrant Name EVERSOURCE ENERGY    
Entity Incorporation, State or Country Code MA    
Entity Address, Address Line One 300 Cadwell Drive    
Entity Address, City or Town Springfield    
Entity Address, State or Province MA    
Entity Address, Postal Zip Code 01104    
City Area Code 800    
Local Phone Number 286-5000    
Entity File Number 1-5324    
Entity Tax Identification Number 04-2147929    
Title of 12(b) Security Common Shares, $5.00 par value per share    
Trading Symbol ES    
Security Exchange Name NYSE    
Entity Well-known Seasoned Issuer Yes    
Entity Voluntary Filers No    
Entity Current Reporting Status Yes    
Entity Interactive Data Current Yes    
Entity Filer Category Large Accelerated Filer    
Entity Small Business false    
Entity Emerging Growth Company false    
ICFR Auditor Attestation Flag true    
Entity Shell Company false    
Entity Public Float     $ 28,496,151,703
Entity Common Stock, Shares Outstanding   343,003,366  
Documents Incorporated by Reference
Portions of the Eversource Energy and Subsidiaries 2019 combined Annual Report on Form 10-K and portions of the Proxy Statement relating to the Annual Meeting of Shareholders to be held on May 5, 2021, are incorporated by reference into Parts II and III of this Report.

NSTAR Electric Company and Public Service Company of New Hampshire each meet the conditions set forth in General Instruction I(1)(a) and (b) of Form 10-K, and each is therefore filing this Form 10-K with the reduced disclosure format specified in General Instruction I(2) of Form 10‑K.  

Eversource Energy, The Connecticut Light and Power Company, NSTAR Electric Company and Public Service Company of New Hampshire each separately file this combined Form 10-K.  Information contained herein relating to any individual registrant is filed by such registrant on its own behalf.  Each registrant makes no representation as to information relating to the other registrants.
    
Entity Central Index Key 0000072741    
Document Fiscal Year Focus 2020    
Document Fiscal Period Focus FY    
Amendment Flag false    
The Connecticut Light and Power Company      
Entity Information [Line Items]      
Current Fiscal Year End Date --12-31    
Entity Registrant Name THE CONNECTICUT LIGHT AND POWER COMPANY    
Entity Incorporation, State or Country Code CT    
Entity Address, Address Line One 107 Selden Street    
Entity Address, City or Town Berlin    
Entity Address, State or Province CT    
Entity Address, Postal Zip Code 06037-1616    
City Area Code 800    
Local Phone Number 286-5000    
Entity File Number 0-00404    
Entity Tax Identification Number 06-0303850    
Entity Filer Category Non-accelerated Filer    
Entity Small Business false    
Entity Emerging Growth Company false    
Entity Shell Company false    
Entity Common Stock, Shares Outstanding   6,035,205  
Entity Central Index Key 0000023426    
NSTAR Electric Company      
Entity Information [Line Items]      
Current Fiscal Year End Date --12-31    
Entity Registrant Name NSTAR ELECTRIC COMPANY    
Entity Incorporation, State or Country Code MA    
Entity Address, Address Line One 800 Boylston Street    
Entity Address, City or Town Boston    
Entity Address, State or Province MA    
Entity Address, Postal Zip Code 02199    
City Area Code 800    
Local Phone Number 286-5000    
Entity File Number 1-02301    
Entity Tax Identification Number 04-1278810    
Entity Filer Category Non-accelerated Filer    
Entity Small Business false    
Entity Emerging Growth Company false    
Entity Shell Company false    
Entity Common Stock, Shares Outstanding   200  
Entity Central Index Key 0000013372    
Public Service Company of New Hampshire      
Entity Information [Line Items]      
Current Fiscal Year End Date --12-31    
Entity Registrant Name PUBLIC SERVICE COMPANY OF NEW HAMPSHIRE    
Entity Incorporation, State or Country Code NH    
Entity Address, Address Line One Energy Park    
Entity Address, Address Line Two 780 North Commercial Street    
Entity Address, City or Town Manchester    
Entity Address, State or Province NH    
Entity Address, Postal Zip Code 03101-1134    
City Area Code 800    
Local Phone Number 286-5000    
Entity File Number 1-6392    
Entity Tax Identification Number 02-0181050    
Entity Filer Category Non-accelerated Filer    
Entity Small Business false    
Entity Emerging Growth Company false    
Entity Shell Company false    
Entity Common Stock, Shares Outstanding   301  
Entity Central Index Key 0000315256