LONG-TERM DEBT - Schedule of Long-term Debt Issuances and Repayments (Details) - USD ($)	1 Months Ended				12 Months Ended
	Nov. 30, 2007	Sep. 30, 2007	Mar. 31, 2007	Sep. 30, 1997	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015	Jan. 31, 2018	Oct. 31, 2017	Sep. 30, 2017	Aug. 31, 2017	May 31, 2017	Mar. 31, 2017
Debt Instrument [Line Items]
Repayments of debt					$ (745,000,000)	$ (200,000,000)	$ (216,700,000)
Senior Notes
Debt Instrument [Line Items]
Long-term debt					11,775,900,000	8,829,400,000
The Connecticut Light And Power Company
Debt Instrument [Line Items]
Repayments of debt					$ (250,000,000)	0	(162,000,000)
The Connecticut Light And Power Company | Senior Notes | 3.20% 2017 Series A First Mortgage Bonds
Debt Instrument [Line Items]
Interest rate stated percentage (as a percentage)					3.20%
Debt instrument, face amount													$ 300,000,000.0
The Connecticut Light And Power Company | Senior Notes | 4.30% Series A First and Refunding Mortgage Bonds
Debt Instrument [Line Items]
Long-term debt					$ 475,000,000
Interest rate stated percentage (as a percentage)					4.30%
Debt instrument, face amount											$ 225,000,000.0
The Connecticut Light And Power Company | Senior Notes | 5.375% 2007 Series A First Mortgage Bonds
Debt Instrument [Line Items]
Repayments of debt			$ (150,000,000)
Interest rate stated percentage (as a percentage)					5.375%
The Connecticut Light And Power Company | Senior Notes | 5.75% 2007 Series C First Mortgage Bonds
Debt Instrument [Line Items]
Repayments of debt		$ (100,000,000)
Interest rate stated percentage (as a percentage)					5.75%
NSTAR Electric Company
Debt Instrument [Line Items]
Repayments of debt					$ (400,000,000)	(200,000,000)	(54,700,000)
NSTAR Electric Company | Unsecured Debt
Debt Instrument [Line Items]
Long-term debt					2,943,800,000	$ 2,244,600,000
NSTAR Electric Company | Unsecured Debt | Unsecured Notes Due 2027
Debt Instrument [Line Items]
Long-term debt					$ 700,000,000
Interest rate stated percentage (as a percentage)					3.20%	3.20%
Debt instrument, face amount									$ 350,000,000.0			$ 350,000,000.0
NSTAR Electric Company | Unsecured Debt | 5.625% Debentures
Debt Instrument [Line Items]
Repayments of debt	$ (400,000,000)
Interest rate stated percentage (as a percentage)					5.625%	5.625%
Public Service Company Of New Hampshire
Debt Instrument [Line Items]
Repayments of debt					$ (70,000,000)	$ 0	$ 0
Public Service Company Of New Hampshire | Senior Notes | 6.15% Series N First Mortgage Bonds
Debt Instrument [Line Items]
Repayments of debt		$ (70,000,000)
Interest rate stated percentage (as a percentage)					6.15%
Yankee Gas Services Company | Senior Notes | Yankee Gas 3.02% Series N First Mortgage Bonds
Debt Instrument [Line Items]
Interest rate stated percentage (as a percentage)					3.02%
Debt instrument, face amount										$ 75,000,000.0
NSTAR Gas Company | Senior Notes | NSTAR Gas 7.04% Series M First Mortgage Bonds
Debt Instrument [Line Items]
Repayments of debt				$ (25,000,000)
Interest rate stated percentage (as a percentage)					7.04%
Parent Company | Senior Notes | Eversource Parent 2.75% Series K Senior Notes
Debt Instrument [Line Items]
Long-term debt					$ 750,000,000
Interest rate stated percentage (as a percentage)					2.75%
Debt instrument, face amount									450,000,000.0				$ 300,000,000.0
Parent Company | Senior Notes | Eversource Parent 2.90% Series L Senior Notes
Debt Instrument [Line Items]
Interest rate stated percentage (as a percentage)					2.90%
Debt instrument, face amount									$ 450,000,000.0
Parent Company | Senior Notes | Eversource Parent 2.50% Series I Senior Notes
Debt Instrument [Line Items]
Long-term debt					$ 450,000,000
Interest rate stated percentage (as a percentage)					2.50%
Parent Company | Senior Notes | Eversource Parent 3.30% Series M Senior Notes
Debt Instrument [Line Items]
Interest rate stated percentage (as a percentage)					3.30%
Parent Company | Subsequent Event | Senior Notes | Eversource Parent 2.50% Series I Senior Notes
Debt Instrument [Line Items]
Debt instrument, face amount								$ 200,000,000.0
Parent Company | Subsequent Event | Senior Notes | Eversource Parent 3.30% Series M Senior Notes
Debt Instrument [Line Items]
Debt instrument, face amount								$ 450,000,000.0
Parent Company | Subsequent Event | Senior Notes | Eversource Parent 1.60% Series G Senior Notes (5)
Debt Instrument [Line Items]
Interest rate stated percentage (as a percentage)								3.30%
Debt instrument, face amount								$ (150,000,000.0)