LONG-TERM DEBT - Narrative (Details) - USD ($) $ in Thousands	Dec. 31, 2025	Mar. 26, 2025	Dec. 31, 2024	Dec. 18, 2024	Aug. 12, 2024	May 01, 2024
Debt Instrument [Line Items]
Long-term debt	$ 26,872,433		$ 25,701,627
Interest included in payment to settle spent nuclear fuel obligation	4,600		4,300
Pre-1983 Spent Nuclear Fuel Obligation (CYAPC)
Debt Instrument [Line Items]
Long-term debt	5,900		5,600
NSTAR Electric
Debt Instrument [Line Items]
Long-term debt	$ 5,645,638		$ 4,844,920
Line of Credit | NSTAR Electric
Debt Instrument [Line Items]
Amount of long-term debt requested for issuance						$ 2,400,000
Line of Credit | EGMA
Debt Instrument [Line Items]
Amount of long-term debt requested for issuance					$ 325,000
Line of Credit | NSTAR Gas
Debt Instrument [Line Items]
Amount of long-term debt requested for issuance				$ 475,000
Line of Credit | Yankee Gas
Debt Instrument [Line Items]
Amount of long-term debt requested for issuance		$ 360,000