LONG-TERM DEBT - Narrative (Details) - USD ($) $ in Thousands	Dec. 31, 2021	Sep. 10, 2021	Mar. 31, 2021	Dec. 31, 2020
Debt Instrument [Line Items]
Long-term debt	$ 17,023,577			$ 15,125,876
Interest included in payment to settle spent nuclear fuel obligation				8,700
Pre-1983 Spent Nuclear Fuel Obligation (CYAPC)
Debt Instrument [Line Items]
Long-term debt	11,700			11,700
NSTAR Electric Company
Debt Instrument [Line Items]
Amount of long-term debt requested for issuance			$ 1,600,000
Long-term debt	$ 3,585,399			$ 3,393,221
Massachusetts Department of Public Utilities
Debt Instrument [Line Items]
Amount of long-term debt requested for issuance		$ 725,000