SHORT-TERM AND LONG-TERM DEBT - Borrowings Outstanding and Available under the Commercial Paper Programs (Details) - Commercial Paper - USD ($) $ in Millions	Mar. 31, 2024	Dec. 31, 2023
Debt Instrument [Line Items]
Borrowings outstanding	$ 1,934.9	$ 1,771.9
Available borrowing capacity	$ 65.1	$ 228.1
Weighted-average interest rate (in percent)	5.66%	5.60%
NSTAR Electric
Debt Instrument [Line Items]
Borrowings outstanding	$ 419.0	$ 365.8
Available borrowing capacity	$ 231.0	$ 284.2
Weighted-average interest rate (in percent)	5.38%	5.40%