COMMON SHARES - Common Shares Authorized and Issued (Details) - $ / shares	Mar. 31, 2021	Dec. 31, 2020	Mar. 31, 2020
Class of Stock [Line Items]
Par value (in dollars per share)	$ 5		$ 5
Authorized (in shares)	380,000,000	380,000,000
Issued (in shares)	357,818,402	357,818,402
CL&P
Class of Stock [Line Items]
Par value (in dollars per share)	$ 10
Authorized (in shares)	24,500,000	24,500,000
Issued (in shares)	6,035,205	6,035,205
NSTAR Electric
Class of Stock [Line Items]
Par value (in dollars per share)	$ 1
Authorized (in shares)	100,000,000	100,000,000
Issued (in shares)	200	200
PSNH
Class of Stock [Line Items]
Par value (in dollars per share)	$ 1
Authorized (in shares)	100,000,000	100,000,000
Issued (in shares)	301	301