LONG-TERM DEBT - Narrative (Details) - USD ($)	1 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Jan. 27, 2020	Dec. 11, 2019	Aug. 14, 2019	Apr. 26, 2019	Feb. 27, 2019
Debt Instrument [Line Items]
Long-Term Debt	$ 13,770,828,000	$ 12,248,743,000
Payment to DOE	29,000,000	145,000,000
Interest included in payment to settle spent nuclear fuel obligation	8,600,000	29,000,000.0
NSTAR Electric
Debt Instrument [Line Items]
Amount of long-term debt approved for issuance							$ 800,000,000
Long-Term Debt	3,247,086,000	2,944,846,000
PSNH
Debt Instrument [Line Items]
Amount of long-term debt approved for issuance						$ 300,000,000
Long-Term Debt	951,620,000	655,173,000
CL&P
Debt Instrument [Line Items]
Amount of long-term debt approved for issuance					$ 675,000,000
Long-Term Debt	3,518,136,000	3,004,016,000
Aquarion
Debt Instrument [Line Items]
Amount of long-term debt approved for issuance				$ 45,000,000
Pre-1983 Spent Nuclear Fuel Obligation (CYAPC)
Debt Instrument [Line Items]
Long-Term Debt	$ 11,600,000	$ 39,500,000
Subsequent Event | NSTAR Gas
Debt Instrument [Line Items]
Amount of long-term debt approved for issuance			$ 270,000,000