LONG-TERM DEBT - Schedule of Long-Term Debt Outstanding (Details) - USD ($)	Dec. 31, 2018	Oct. 31, 2018	Sep. 30, 2018	Mar. 31, 2018	Jan. 31, 2018	Jan. 08, 2018	Dec. 31, 2017	Dec. 08, 2017	Jan. 31, 2015	May 31, 2013	May 31, 2008
Debt Instrument [Line Items]
Long-Term Debt	$ 12,248,743,000						$ 11,775,889,000
Less Amounts due Within One Year	$ (837,319,000)						(549,631,000)
Eversource Parent 2.50% Series I Senior Notes (2)
Debt Instrument [Line Items]
Interest rate stated percentage (up to) (as a percentage)					2.50%
Eversource Parent 3.30% Series M Senior Notes
Debt Instrument [Line Items]
Interest rate stated percentage (up to) (as a percentage)					3.30%
Eversource Parent 3.80% Series N Senior Notes
Debt Instrument [Line Items]
Interest rate stated percentage (up to) (as a percentage)	3.80%
Eversource Parent 4.25% Series O Senior Notes
Debt Instrument [Line Items]
Interest rate stated percentage (up to) (as a percentage)	4.25%
Eversource Parent 1.60% Series G Senior Notes
Debt Instrument [Line Items]
Interest rate stated percentage (up to) (as a percentage)									1.60%
Eversource Parent 1.45% Series E Senior Notes
Debt Instrument [Line Items]
Interest rate stated percentage (up to) (as a percentage)										1.45%
First Mortgage Notes | Yankee Gas - First Mortgage Bonds: 3.020% - 8.480% due 2019 - 2048
Debt Instrument [Line Items]
Long-Term Debt	$ 470,000,000						520,000,000
First Mortgage Notes | NSTAR Gas - First Mortgage Bonds: 4.09% - 9.950% due 2020 - 2048
Debt Instrument [Line Items]
Long-Term Debt	385,000,000						285,000,000
Unsecured Debt | 4.500% Debentures due 2019
Debt Instrument [Line Items]
Long-Term Debt	350,000,000						350,000,000
Unsecured Debt | 2.500% - 4.250% Senior Notes due 2021 - 2029
Debt Instrument [Line Items]
Long-Term Debt	4,360,000,000						3,260,000,000
Senior Notes
Debt Instrument [Line Items]
Unamortized Premiums and Discounts, Net	4,200,000						(1,500,000)
Unamortized Debt Issuance Costs	(23,200,000)						(12,800,000)
Long-term debt	12,248,700,000						11,775,900,000
Loans Payable
Debt Instrument [Line Items]
Less Amounts due Within One Year	(401,100,000)						(104,200,000)
Loans Payable | Unsecured Notes 3.570% - 6.430% due 2021 - 2037
Debt Instrument [Line Items]
Long-Term Debt	289,500,000						290,900,000
Loans Payable | Secured Debt 4.100% - 9.640% due 2021 - 2035
Debt Instrument [Line Items]
Long-Term Debt	70,700,000						70,400,000
Pre-1983 Spent Nuclear Fuel Obligation (CYAPC)
Debt Instrument [Line Items]
Long-Term Debt	39,500,000						181,400,000
Fair Value Adjustment
Debt Instrument [Line Items]
Long-Term Debt	144,700,000						172,600,000
Fair Value Adjustment - Current Portion
Debt Instrument [Line Items]
Long-Term Debt	(36,200,000)						(35,400,000)
Total Other Long-Term Debt
Debt Instrument [Line Items]
Long-Term Debt	5,644,700,000						5,180,600,000
The Connecticut Light And Power Company
Debt Instrument [Line Items]
Long-Term Debt	3,004,016,000						2,759,135,000
Less Amounts due Within One Year	(250,000,000)						(300,000,000)
The Connecticut Light And Power Company | 5.650% 2008 Series A due 2018
Debt Instrument [Line Items]
Interest rate stated percentage (up to) (as a percentage)											5.65%
The Connecticut Light And Power Company | 4.000% 2018 Series A due 2048
Debt Instrument [Line Items]
Interest rate stated percentage (up to) (as a percentage)				4.00%
The Connecticut Light And Power Company | 4.375% Fixed Rate Tax Exempt due 2028
Debt Instrument [Line Items]
Long-Term Debt	120,500,000						120,500,000
The Connecticut Light And Power Company | First Mortgage Notes
Debt Instrument [Line Items]
Long-Term Debt	3,144,800,000						2,944,800,000
Unamortized Premiums and Discounts, Net	10,200,000						11,500,000
Unamortized Debt Issuance Costs	(21,500,000)						(17,700,000)
Long-term debt	$ 3,004,000,000						2,759,100,000
The Connecticut Light And Power Company | First Mortgage Notes | 7.875% 1994 Series D due 2024
Debt Instrument [Line Items]
Interest rate stated percentage (up to) (as a percentage)	7.875%
Long-Term Debt	$ 139,800,000						139,800,000
The Connecticut Light And Power Company | First Mortgage Notes | 5.750% 2004 Series B due 2034
Debt Instrument [Line Items]
Interest rate stated percentage (up to) (as a percentage)	5.75%
Long-Term Debt	$ 130,000,000						130,000,000
The Connecticut Light And Power Company | First Mortgage Notes | 5.625% 2005 Series B due 2035
Debt Instrument [Line Items]
Interest rate stated percentage (up to) (as a percentage)	5.625%
Long-Term Debt	$ 100,000,000						100,000,000
The Connecticut Light And Power Company | First Mortgage Notes | 6.350% 2006 Series A due 2036
Debt Instrument [Line Items]
Interest rate stated percentage (up to) (as a percentage)	6.35%
Long-Term Debt	$ 250,000,000						250,000,000
The Connecticut Light And Power Company | First Mortgage Notes | 5.750% 2007 Series B due 2037
Debt Instrument [Line Items]
Interest rate stated percentage (up to) (as a percentage)	5.75%
Long-Term Debt	$ 150,000,000						150,000,000
The Connecticut Light And Power Company | First Mortgage Notes | 6.375% 2007 Series D due 2037
Debt Instrument [Line Items]
Interest rate stated percentage (up to) (as a percentage)	6.375%
Long-Term Debt	$ 100,000,000						100,000,000
The Connecticut Light And Power Company | First Mortgage Notes | 5.650% 2008 Series A due 2018
Debt Instrument [Line Items]
Interest rate stated percentage (up to) (as a percentage)	5.65%
Long-Term Debt	$ 0						300,000,000
The Connecticut Light And Power Company | First Mortgage Notes | 5.500% 2009 Series A due 2019
Debt Instrument [Line Items]
Interest rate stated percentage (up to) (as a percentage)	5.50%
Long-Term Debt	$ 250,000,000						250,000,000
The Connecticut Light And Power Company | First Mortgage Notes | 2.500% 2013 Series A due 2023
Debt Instrument [Line Items]
Interest rate stated percentage (up to) (as a percentage)	2.50%
Long-Term Debt	$ 400,000,000						400,000,000
The Connecticut Light And Power Company | First Mortgage Notes | 4.300% 2014 Series A due 2044
Debt Instrument [Line Items]
Interest rate stated percentage (up to) (as a percentage)	4.30%
Long-Term Debt	$ 475,000,000						475,000,000
The Connecticut Light And Power Company | First Mortgage Notes | 4.150% 2015 Series A due 2045
Debt Instrument [Line Items]
Interest rate stated percentage (up to) (as a percentage)	4.15%
Long-Term Debt	$ 350,000,000						350,000,000
The Connecticut Light And Power Company | First Mortgage Notes | 3.200% 2017 Series A due 2027
Debt Instrument [Line Items]
Interest rate stated percentage (up to) (as a percentage)	3.20%
Long-Term Debt	$ 300,000,000						300,000,000
The Connecticut Light And Power Company | First Mortgage Notes | 2018 Series A First Mortgage Bonds Due 2048 [Member]
Debt Instrument [Line Items]
Long-Term Debt	$ 500,000,000						0
The Connecticut Light And Power Company | First Mortgage Notes | 4.000% 2018 Series A due 2048
Debt Instrument [Line Items]
Interest rate stated percentage (up to) (as a percentage)	4.00%
The Connecticut Light And Power Company | First Mortgage Notes | 4.375% Fixed Rate Tax Exempt due 2028
Debt Instrument [Line Items]
Interest rate stated percentage (up to) (as a percentage)	4.375%
NSTAR Electric Company
Debt Instrument [Line Items]
Long-Term Debt	$ 2,944,846,000						2,943,759,000
NSTAR Electric Company | NSTAR Gas 4.09% Series P First Mortgage Bonds
Debt Instrument [Line Items]
Interest rate stated percentage (up to) (as a percentage)			4.09%
NSTAR Electric Company | Unsecured Debt
Debt Instrument [Line Items]
Long-Term Debt	2,400,000,000						2,400,000,000
Less Amounts due Within One Year	0						0
Unamortized Premiums and Discounts, Net	2,500,000						1,800,000
Unamortized Debt Issuance Costs	(17,700,000)						(19,400,000)
Long-term debt	$ 2,944,800,000						2,943,800,000
NSTAR Electric Company | Unsecured Debt | 5.750% due 2036
Debt Instrument [Line Items]
Interest rate stated percentage (up to) (as a percentage)	5.75%
Long-Term Debt	$ 200,000,000						200,000,000
NSTAR Electric Company | Unsecured Debt | 5.500% due 2040
Debt Instrument [Line Items]
Interest rate stated percentage (up to) (as a percentage)	5.50%
Long-Term Debt	$ 300,000,000						300,000,000
NSTAR Electric Company | Unsecured Debt | 2.375% due 2022
Debt Instrument [Line Items]
Interest rate stated percentage (up to) (as a percentage)	2.375%
Long-Term Debt	$ 400,000,000						400,000,000
NSTAR Electric Company | Unsecured Debt | 4.400% due 2044
Debt Instrument [Line Items]
Interest rate stated percentage (up to) (as a percentage)	4.40%
Long-Term Debt	$ 300,000,000						300,000,000
NSTAR Electric Company | Unsecured Debt | 3.250% due 2025
Debt Instrument [Line Items]
Interest rate stated percentage (up to) (as a percentage)	3.25%
Long-Term Debt	$ 250,000,000						250,000,000
NSTAR Electric Company | Unsecured Debt | 2.700% due 2026
Debt Instrument [Line Items]
Interest rate stated percentage (up to) (as a percentage)	2.70%
Long-Term Debt	$ 250,000,000						250,000,000
NSTAR Electric Company | Unsecured Debt | 3.200% due 2027
Debt Instrument [Line Items]
Interest rate stated percentage (up to) (as a percentage)	3.20%
Long-Term Debt	$ 700,000,000						700,000,000
NSTAR Electric Company | Senior Notes
Debt Instrument [Line Items]
Long-Term Debt	$ 565,000,000						565,000,000
NSTAR Electric Company | Senior Notes | 5.900% Senior Notes Series B due 2034
Debt Instrument [Line Items]
Interest rate stated percentage (up to) (as a percentage)	5.90%
Long-Term Debt	$ 50,000,000						50,000,000
NSTAR Electric Company | Senior Notes | 6.700% Senior Notes Series D due 2037
Debt Instrument [Line Items]
Interest rate stated percentage (up to) (as a percentage)	6.70%
Long-Term Debt	$ 40,000,000						40,000,000
NSTAR Electric Company | Senior Notes | 5.100% Senior Notes Series E due 2020
Debt Instrument [Line Items]
Interest rate stated percentage (up to) (as a percentage)	5.10%
Long-Term Debt	$ 95,000,000						95,000,000
NSTAR Electric Company | Senior Notes | 3.500% Senior Notes Series F due 2021
Debt Instrument [Line Items]
Interest rate stated percentage (up to) (as a percentage)	3.50%
Long-Term Debt	$ 250,000,000						250,000,000
NSTAR Electric Company | Senior Notes | 3.880% Senior Notes Series G due 2023
Debt Instrument [Line Items]
Interest rate stated percentage (up to) (as a percentage)	3.88%
Long-Term Debt	$ 80,000,000						80,000,000
NSTAR Electric Company | Senior Notes | 2.750% Senior Notes Series H due 2026
Debt Instrument [Line Items]
Interest rate stated percentage (up to) (as a percentage)	2.75%
Long-Term Debt	$ 50,000,000						50,000,000
Public Service Company Of New Hampshire
Debt Instrument [Line Items]
Long-Term Debt	655,173,000						892,438,000
Less Amounts due Within One Year	(150,000,000)						(110,000,000)
Public Service Company Of New Hampshire | 6.000% Series O due 2018
Debt Instrument [Line Items]
Interest rate stated percentage (up to) (as a percentage)											6.00%
Public Service Company Of New Hampshire | First Mortgage Notes
Debt Instrument [Line Items]
Long-Term Debt	807,000,000						917,000,000
Less Amounts due Within One Year	(150,000,000)						(110,000,000)
Unamortized Premiums and Discounts, Net	0						(200,000)
Unamortized Debt Issuance Costs	(1,800,000)						(4,100,000)
Long-term debt	$ 655,200,000						892,400,000
Public Service Company Of New Hampshire | First Mortgage Notes | 5.600% Series M due 2035
Debt Instrument [Line Items]
Interest rate stated percentage (up to) (as a percentage)	5.60%
Long-Term Debt	$ 50,000,000						50,000,000
Public Service Company Of New Hampshire | First Mortgage Notes | 6.000% Series O due 2018
Debt Instrument [Line Items]
Interest rate stated percentage (up to) (as a percentage)	6.00%
Long-Term Debt	$ 0						110,000,000
Public Service Company Of New Hampshire | First Mortgage Notes | 4.500% Series P due 2019
Debt Instrument [Line Items]
Interest rate stated percentage (up to) (as a percentage)	4.50%
Long-Term Debt	$ 150,000,000						150,000,000
Public Service Company Of New Hampshire | First Mortgage Notes | 4.050% Series Q due 2021
Debt Instrument [Line Items]
Interest rate stated percentage (up to) (as a percentage)	4.05%
Long-Term Debt	$ 122,000,000						122,000,000
Public Service Company Of New Hampshire | First Mortgage Notes | 3.200% Series R due 2021
Debt Instrument [Line Items]
Interest rate stated percentage (up to) (as a percentage)	3.20%
Long-Term Debt	$ 160,000,000						160,000,000
Public Service Company Of New Hampshire | First Mortgage Notes | 3.500% Series S due 2023
Debt Instrument [Line Items]
Interest rate stated percentage (up to) (as a percentage)	3.50%
Long-Term Debt	$ 325,000,000						325,000,000
Public Service Company Of New Hampshire | First Mortgage Notes | Adjustable Rate Tax Exempt Series A Notes Due 2021 [Member]
Debt Instrument [Line Items]
Long-Term Debt	0						89,300,000
Yankee Gas Services Company | Yankee Gas 4.13% Series O First Mortgage Bonds
Debt Instrument [Line Items]
Interest rate stated percentage (up to) (as a percentage)			4.13%
Yankee Gas Services Company | Yankee Gas 6.90% Series J First Mortgage Bonds
Debt Instrument [Line Items]
Interest rate stated percentage (up to) (as a percentage)		6.90%
Commercial Paper
Debt Instrument [Line Items]
Commercial Paper Classified as Long-Term Debt	0						201,200,000.0
Commercial Paper | NSTAR Electric Company
Debt Instrument [Line Items]
Commercial Paper Classified as Long-Term Debt	$ 650,000,000
Minimum | First Mortgage Notes | Yankee Gas - First Mortgage Bonds: 3.020% - 8.480% due 2019 - 2048
Debt Instrument [Line Items]
Interest rate stated percentage (up to) (as a percentage)	3.02%
Minimum | First Mortgage Notes | NSTAR Gas - First Mortgage Bonds: 4.09% - 9.950% due 2020 - 2048
Debt Instrument [Line Items]
Interest rate stated percentage (up to) (as a percentage)	4.35%
Minimum | Unsecured Debt | 4.500% Debentures due 2019
Debt Instrument [Line Items]
Interest rate stated percentage (up to) (as a percentage)	4.50%
Minimum | Unsecured Debt | 2.500% - 4.250% Senior Notes due 2021 - 2029
Debt Instrument [Line Items]
Interest rate stated percentage (up to) (as a percentage)	2.50%
Minimum | Loans Payable | Unsecured Notes 3.570% - 6.430% due 2021 - 2037
Debt Instrument [Line Items]
Interest rate stated percentage (up to) (as a percentage)	3.57%
Minimum | Loans Payable | Secured Debt 4.100% - 9.640% due 2021 - 2035
Debt Instrument [Line Items]
Interest rate stated percentage (up to) (as a percentage)	4.10%
Parent Company
Debt Instrument [Line Items]
Long-Term Debt	$ 4,031,997,000						3,523,472,000
Less Amounts due Within One Year	(378,883,000)						$ (32,114,000)
Parent Company | Senior Notes | Eversource Parent 2.50% Series I Senior Notes (2)
Debt Instrument [Line Items]
Long-term debt	450,000,000
Parent Company | Commercial Paper
Debt Instrument [Line Items]
Less Amounts due Within One Year					$ (446,800,000)
Commercial Paper Classified as Long-Term Debt	$ 1,450,000,000					$ 201,200,000.0		$ 1,450,000,000.00