LONG-TERM DEBT - Narrative (Details) - USD ($)	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015	Jan. 31, 2018	Jan. 08, 2018	Dec. 20, 2017	Dec. 08, 2017	Mar. 30, 2017	Jan. 04, 2017
Debt Instrument [Line Items]
Repayments of Long-term Debt	$ 745,000,000	$ 200,000,000	$ 216,700,000
Current portion of long-term debt	549,631,000	773,883,000
Long-term debt outstanding	11,775,889,000	8,829,354,000
Interest included in payment to settle spent nuclear fuel obligation	132,600,000	131,200,000
The Connecticut Light And Power Company
Debt Instrument [Line Items]
Repayments of Long-term Debt	250,000,000	0	162,000,000
Current portion of long-term debt	300,000,000	250,000,000
Long-term debt outstanding	2,759,135,000	2,516,010,000
NSTAR Electric Company
Debt Instrument [Line Items]
Repayments of Long-term Debt	400,000,000	200,000,000	54,700,000
Current portion of long-term debt	0	400,000,000
Long-term debt outstanding	2,943,759,000	2,244,653,000
Public Service Company Of New Hampshire
Debt Instrument [Line Items]
Repayments of Long-term Debt	70,000,000	0	$ 0
Current portion of long-term debt	110,000,000	70,000,000
Long-term debt outstanding	892,438,000	1,002,048,000
Parent Company
Debt Instrument [Line Items]
Current portion of long-term debt	32,114,000	28,883,000
Long-term debt outstanding	3,523,472,000	2,158,519,000
Commercial Paper
Debt Instrument [Line Items]
Amount of commercial paper program	201,200,000.0	0
Commercial Paper | NSTAR Electric Company
Debt Instrument [Line Items]
Amount of commercial paper program	650,000,000						$ 450,000,000
Commercial Paper | Parent Company
Debt Instrument [Line Items]
Amount of commercial paper program	1,450,000,000						$ 1,450,000,000.00
Commercial Paper | Parent Company | Subsequent Event
Debt Instrument [Line Items]
Current portion of long-term debt				$ 446,800,000
Amount of commercial paper program					$ 201,200,000.0
Senior Notes | The Connecticut Light And Power Company
Debt Instrument [Line Items]
Amount of debt authorized									$ 1,325,000,000
Senior Notes | NSTAR Electric Company
Debt Instrument [Line Items]
Amount of debt authorized								$ 700,000,000
Long-term debt outstanding	565,000,000	565,000,000
Senior Notes | Yankee Gas Services Company
Debt Instrument [Line Items]
Amount of debt authorized						$ 50,000,000
Pre-1983 Spent Nuclear Fuel Obligation (CYAPC)
Debt Instrument [Line Items]
Long-term debt outstanding	$ 181,400,000	$ 180,000,000
Adjustable Rate Tax Exempt Series A Notes Due 2021 | Senior Notes | Public Service Company Of New Hampshire
Debt Instrument [Line Items]
Par value of stock (as a percentage)	100.00%