SHORT-TERM AND LONG-TERM DEBT - Borrowings Outstanding and Available under the Commercial Paper Programs (Details) - Commercial Paper - USD ($) $ in Millions	Jun. 30, 2024	Dec. 31, 2023
Debt Instrument [Line Items]
Borrowings outstanding	$ 1,203.0	$ 1,771.9
Available borrowing capacity	$ 797.0	$ 228.1
Weighted-average interest rate (in percent)	5.67%	5.60%
NSTAR Electric
Debt Instrument [Line Items]
Borrowings outstanding	$ 236.5	$ 365.8
Available borrowing capacity	$ 413.5	$ 284.2
Weighted-average interest rate (in percent)	5.37%	5.40%