PREFERRED STOCK NOT SUBJECT TO MANDATORY REDEMPTION - Narrative (Details)	Dec. 31, 2017 $ / shares shares
The Connecticut Light And Power Company
Class of Stock [Line Items]
Preferred stock, shares authorized (in shares) | shares	9,000,000
Preferred stock par value per share (in dollars per share) | $ / shares	$ 50
NSTAR Electric Company
Class of Stock [Line Items]
Preferred stock, shares authorized (in shares) | shares	2,890,000
Preferred stock par value per share (in dollars per share) | $ / shares	$ 100