SHORT-TERM AND LONG-TERM DEBT - Borrowings Outstanding and Available under the Commercial Paper Programs (Details) - Commercial Paper - USD ($) $ in Millions	Sep. 30, 2025	Dec. 31, 2024
Debt Instrument [Line Items]
Borrowings outstanding	$ 1,038.5	$ 1,538.0
Available borrowing capacity	$ 961.5	$ 462.0
Weighted-average interest rate (in percent)	4.50%	4.76%
NSTAR Electric
Debt Instrument [Line Items]
Borrowings outstanding	$ 479.0	$ 504.8
Available borrowing capacity	$ 171.0	$ 145.2
Weighted-average interest rate (in percent)	4.30%	4.55%