SHORT-TERM AND LONG-TERM DEBT - Borrowings Outstanding and Available under the Commercial Paper Programs (Details) - Commercial Paper - USD ($) $ in Millions	Sep. 30, 2023	Dec. 31, 2022
Debt Instrument [Line Items]
Borrowings outstanding	$ 1,320.3	$ 1,442.2
Available borrowing capacity	$ 679.7	$ 557.8
Weighted-average interest rate (in percent)	5.53%	4.63%
NSTAR Electric
Debt Instrument [Line Items]
Borrowings outstanding	$ 209.5	$ 0.0
Available borrowing capacity	$ 440.5	$ 650.0
Weighted-average interest rate (in percent)	5.35%	0.00%