COMMON SHARES - Common Shares Authorized and Issued (Details) - $ / shares	Sep. 30, 2019	Jun. 30, 2019	Dec. 31, 2018
Class of Stock [Line Items]
Per value (in dollars per share)	$ 5	$ 5
Authorized (in shares)	380,000,000		380,000,000
Issued (in shares)	339,858,402		333,878,402
CL&P
Class of Stock [Line Items]
Per 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]
Per 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]
Per value (in dollars per share)	$ 1
Authorized (in shares)	100,000,000		100,000,000
Issued (in shares)	301		301