SHORT-TERM AND LONG-TERM DEBT - Borrowings Outstanding and Available under the Commercial Paper Programs (Details) - Commercial Paper - USD ($) $ in Millions	Sep. 30, 2024	Dec. 31, 2023
Debt Instrument [Line Items]
Borrowings outstanding	$ 322.0	$ 1,771.9
Available borrowing capacity	$ 1,678.0	$ 228.1
Weighted-average interest rate (in percent)	5.12%	5.60%
NSTAR Electric
Debt Instrument [Line Items]
Borrowings outstanding	$ 537.5	$ 365.8
Available borrowing capacity	$ 112.5	$ 284.2
Weighted-average interest rate (in percent)	4.94%	5.40%