SHORT-TERM AND LONG-TERM DEBT - Borrowings Outstanding and Available under the Commercial Paper Programs (Details) - Commercial Paper - USD ($) $ in Millions	Mar. 31, 2025	Dec. 31, 2024
Debt Instrument [Line Items]
Borrowings outstanding	$ 1,381.6	$ 1,538.0
Available borrowing capacity	$ 618.4	$ 462.0
Weighted-average interest rate (in percent)	4.78%	4.76%
NSTAR Electric
Debt Instrument [Line Items]
Borrowings outstanding	$ 108.5	$ 504.8
Available borrowing capacity	$ 541.5	$ 145.2
Weighted-average interest rate (in percent)	4.43%	4.55%