LONG-TERM DEBT - Narrative (Details)	1 Months Ended		12 Months Ended
	Dec. 31, 2018 USD ($)	Jan. 31, 2018 USD ($) issuance	Dec. 31, 2018 USD ($)	Dec. 31, 2017 USD ($)	Dec. 31, 2016 USD ($)	Aug. 01, 2018 USD ($)	Jan. 08, 2018 USD ($)	Dec. 08, 2017 USD ($)
Debt Instrument [Line Items]
Repayments of Long-term Debt			$ 1,050,330,000	$ 745,000,000	$ 200,000,000
Current portion of long-term debt	$ 837,319,000		837,319,000	549,631,000
Amount of debt authorized						$ 800,000,000
Long-term debt outstanding	12,248,743,000		12,248,743,000	11,775,889,000
Payment to DOE	145,000,000
Interest included in payment to settle spent nuclear fuel obligation	29,000,000		29,000,000	132,600,000
The Connecticut Light And Power Company
Debt Instrument [Line Items]
Repayments of Long-term Debt			300,000,000	250,000,000	0
Current portion of long-term debt	250,000,000		250,000,000	300,000,000
Long-term debt outstanding	3,004,016,000		3,004,016,000	2,759,135,000
NSTAR Electric Company
Debt Instrument [Line Items]
Repayments of Long-term Debt			0	400,000,000	200,000,000
Amount of debt authorized						$ 200,000,000
Long-term debt outstanding	2,944,846,000		2,944,846,000	2,943,759,000
Public Service Company Of New Hampshire
Debt Instrument [Line Items]
Repayments of Long-term Debt			199,250,000	70,000,000	0
Current portion of long-term debt	150,000,000		150,000,000	110,000,000
Long-term debt outstanding	655,173,000		655,173,000	892,438,000
Parent Company
Debt Instrument [Line Items]
Repayments of Long-term Debt			450,000,000	0	$ 0
Current portion of long-term debt	378,883,000		378,883,000	32,114,000
Long-term debt outstanding	4,031,997,000		4,031,997,000	3,523,472,000
Commercial Paper
Debt Instrument [Line Items]
Amount of commercial paper program	0		0	201,200,000.0
Commercial Paper | NSTAR Electric Company
Debt Instrument [Line Items]
Amount of commercial paper program	650,000,000		650,000,000
Commercial Paper | Parent Company
Debt Instrument [Line Items]
Current portion of long-term debt		$ 446,800,000
Number of Debt Issuances | issuance		2
Amount of commercial paper program	1,450,000,000		1,450,000,000				$ 201,200,000.0	$ 1,450,000,000.00
Senior Notes
Debt Instrument [Line Items]
Long-term debt	12,248,700,000		12,248,700,000	11,775,900,000
Senior Notes | NSTAR Electric Company
Debt Instrument [Line Items]
Long-term debt outstanding	565,000,000		565,000,000	565,000,000
Pre-1983 Spent Nuclear Fuel Obligation (CYAPC)
Debt Instrument [Line Items]
Long-term debt outstanding	$ 39,500,000		$ 39,500,000	$ 181,400,000