PREFERRED STOCK NOT SUBJECT TO MANDATORY REDEMPTION - Narrative (Details)	Dec. 31, 2019 $ / shares shares
CL&P
Class of Stock [Line Items]
Preferred stock, shares authorized (in shares) | shares	9,000,000
Preferred stock, par value (in dollars per share) | $ / shares	$ 50
NSTAR Electric
Class of Stock [Line Items]
Preferred stock, shares authorized (in shares) | shares	2,890,000
Preferred stock, par value (in dollars per share) | $ / shares	$ 100