COMMON SHARES - Schedule of Common Shares (Details) - $ / shares	Mar. 31, 2018	Dec. 31, 2017
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)	333,878,402	333,878,402
The Connecticut Light And Power Company
Class of Stock [Line Items]
Per Value (in dollars per share)	$ 10	$ 10
Authorized (in shares)	24,500,000	24,500,000
Issued (in shares)	6,035,205	6,035,205
NSTAR Electric Company
Class of Stock [Line Items]
Per Value (in dollars per share)	$ 1	$ 1
Authorized (in shares)	100,000,000	100,000,000
Issued (in shares)	200	200
Public Service Company Of New Hampshire
Class of Stock [Line Items]
Per Value (in dollars per share)	$ 1	$ 1
Authorized (in shares)	100,000,000	100,000,000
Issued (in shares)	301	301