LONG-TERM DEBT - Schedule of Long-Term Debt Outstanding (Details) - USD ($)	Dec. 31, 2017	Dec. 08, 2017	Oct. 31, 2017	Aug. 31, 2017	May 31, 2017	Mar. 31, 2017	Dec. 31, 2016
Debt Instrument [Line Items]
Long-Term Debt	$ 11,775,889,000						$ 8,829,354,000
Less Amounts due Within One Year	(549,631,000)						(773,883,000)
First Mortgage Notes | NSTAR Gas - First Mortgage Bonds: 4.350% - 9.950% due 2020 - 2045
Debt Instrument [Line Items]
Long-Term Debt	285,000,000						310,000,000
First Mortgage Notes | Yankee Gas - First Mortgage Bonds: 3.020% - 8.480% due 2018 - 2044
Debt Instrument [Line Items]
Long-Term Debt	520,000,000						445,000,000
Unsecured Debt | 4.500% Debentures due 2019
Debt Instrument [Line Items]
Long-Term Debt	350,000,000						350,000,000
Unsecured Debt | 1.450% - 4.000% Senior Notes due 2018 - 2026
Debt Instrument [Line Items]
Long-Term Debt	3,260,000,000						1,700,000,000
Senior Notes
Debt Instrument [Line Items]
Unamortized Premiums and Discounts, Net	1,500,000						(1,800,000)
Unamortized Debt Issuance Costs	(12,800,000)						(7,100,000)
Long-term debt	11,775,900,000						8,829,400,000
Loans Payable
Debt Instrument [Line Items]
Less Amounts due Within One Year	(104,200,000)						(25,000,000)
Loans Payable | Notes Payable Unsecured 3.57% - 6.430% due 2021 - 2037
Debt Instrument [Line Items]
Long-Term Debt	290,900,000						0
Loans Payable | Notes Payable Secured 4.10% - 9.64% due 2021 - 2035
Debt Instrument [Line Items]
Long-Term Debt	70,400,000						0
Pre-1983 Spent Nuclear Fuel Obligation (CYAPC)
Debt Instrument [Line Items]
Long-Term Debt	181,400,000						180,000,000
Fair Value Adjustment
Debt Instrument [Line Items]
Long-Term Debt	172,600,000						144,600,000
Fair Value Adjustment - Current Portion
Debt Instrument [Line Items]
Long-Term Debt	(35,400,000)						(28,900,000)
Total Other Long-Term Debt
Debt Instrument [Line Items]
Long-Term Debt	5,180,600,000						3,066,800,000
The Connecticut Light And Power Company
Debt Instrument [Line Items]
Long-Term Debt	2,759,135,000						2,516,010,000
Less Amounts due Within One Year	(300,000,000)						(250,000,000)
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	2,944,800,000						2,669,800,000
Unamortized Premiums and Discounts, Net	11,500,000						(10,000,000)
Unamortized Debt Issuance Costs	(17,700,000)						(14,300,000)
Long-term debt	2,759,100,000						2,516,000,000
The Connecticut Light And Power Company | First Mortgage Notes | 7.875% 1994 Series D due 2024
Debt Instrument [Line Items]
Long-Term Debt	$ 139,800,000						$ 139,800,000
Interest rate stated percentage (as a percentage)	7.875%						7.875%
The Connecticut Light And Power Company | First Mortgage Notes | 5.750% 2004 Series B due 2034
Debt Instrument [Line Items]
Long-Term Debt	$ 130,000,000						$ 130,000,000
Interest rate stated percentage (as a percentage)	5.75%						5.75%
The Connecticut Light And Power Company | First Mortgage Notes | 5.625% 2005 Series B due 2035
Debt Instrument [Line Items]
Long-Term Debt	$ 100,000,000						$ 100,000,000
Interest rate stated percentage (as a percentage)	5.625%						5.625%
The Connecticut Light And Power Company | First Mortgage Notes | 6.350% 2006 Series A due 2036
Debt Instrument [Line Items]
Long-Term Debt	$ 250,000,000						$ 250,000,000
Interest rate stated percentage (as a percentage)	6.35%						6.35%
The Connecticut Light And Power Company | First Mortgage Notes | 5.375% 2007 Series A due 2017
Debt Instrument [Line Items]
Long-Term Debt	$ 0						$ 150,000,000
Interest rate stated percentage (as a percentage)	5.375%						5.375%
The Connecticut Light And Power Company | First Mortgage Notes | 5.750% 2007 Series B due 2037
Debt Instrument [Line Items]
Long-Term Debt	$ 150,000,000						$ 150,000,000
Interest rate stated percentage (as a percentage)	5.75%						5.75%
The Connecticut Light And Power Company | First Mortgage Notes | 5.750% 2007 Series C due 2017
Debt Instrument [Line Items]
Long-Term Debt	$ 0						$ 100,000,000
Interest rate stated percentage (as a percentage)	5.75%						5.75%
The Connecticut Light And Power Company | First Mortgage Notes | 6.375% 2007 Series D due 2037
Debt Instrument [Line Items]
Long-Term Debt	$ 100,000,000						$ 100,000,000
Interest rate stated percentage (as a percentage)	6.375%						6.375%
The Connecticut Light And Power Company | First Mortgage Notes | 5.650% 2008 Series A due 2018
Debt Instrument [Line Items]
Long-Term Debt	$ 300,000,000						$ 300,000,000
Interest rate stated percentage (as a percentage)	5.65%						5.65%
The Connecticut Light And Power Company | First Mortgage Notes | 5.500% 2009 Series A due 2019
Debt Instrument [Line Items]
Long-Term Debt	$ 250,000,000						$ 250,000,000
Interest rate stated percentage (as a percentage)	5.50%						5.50%
The Connecticut Light And Power Company | First Mortgage Notes | 2.500% 2013 Series A due 2023
Debt Instrument [Line Items]
Long-Term Debt	$ 400,000,000						$ 400,000,000
Interest rate stated percentage (as a percentage)	2.50%						2.50%
The Connecticut Light And Power Company | First Mortgage Notes | 4.300% 2014 Series A due 2044
Debt Instrument [Line Items]
Long-Term Debt	$ 475,000,000						$ 250,000,000
Interest rate stated percentage (as a percentage)	4.30%						4.30%
The Connecticut Light And Power Company | First Mortgage Notes | 4.150% 2015 Series A due 2045
Debt Instrument [Line Items]
Long-Term Debt	$ 350,000,000						$ 350,000,000
Interest rate stated percentage (as a percentage)	4.15%						4.15%
The Connecticut Light And Power Company | First Mortgage Notes | 3.200% 2017 Series A due 2027
Debt Instrument [Line Items]
Long-Term Debt	$ 300,000,000						$ 0
Interest rate stated percentage (as a percentage)	3.20%						3.20%
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 (as a percentage)	4.375%						4.375%
The Connecticut Light And Power Company | Senior Notes | 5.750% 2007 Series C due 2017
Debt Instrument [Line Items]
Interest rate stated percentage (as a percentage)	5.75%
The Connecticut Light And Power Company | Senior Notes | 4.300% 2014 Series A due 2044
Debt Instrument [Line Items]
Long-term debt	$ 475,000,000
Debt instrument, face amount				$ 225,000,000.0
Interest rate stated percentage (as a percentage)	4.30%
NSTAR Electric Company
Debt Instrument [Line Items]
Long-Term Debt	$ 2,943,759,000						$ 2,244,653,000
Less Amounts due Within One Year	0						(400,000,000)
NSTAR Electric Company | Unsecured Debt
Debt Instrument [Line Items]
Long-Term Debt	2,400,000,000						2,100,000,000
Less Amounts due Within One Year	0						(400,000,000)
Unamortized Premiums and Discounts, Net	(1,800,000)						(4,900,000)
Unamortized Debt Issuance Costs	(19,400,000)						(15,500,000)
Long-term debt	2,943,800,000						2,244,600,000
NSTAR Electric Company | Unsecured Debt | 5.750% due 2036
Debt Instrument [Line Items]
Long-Term Debt	$ 200,000,000						$ 200,000,000
Interest rate stated percentage (as a percentage)	5.75%						5.75%
NSTAR Electric Company | Unsecured Debt | 5.625% due 2017
Debt Instrument [Line Items]
Long-Term Debt	$ 0						$ 400,000,000
Interest rate stated percentage (as a percentage)	5.625%						5.625%
NSTAR Electric Company | Unsecured Debt | 5.500% due 2040
Debt Instrument [Line Items]
Long-Term Debt	$ 300,000,000						$ 300,000,000
Interest rate stated percentage (as a percentage)	5.50%						5.50%
NSTAR Electric Company | Unsecured Debt | 2.375% due 2022
Debt Instrument [Line Items]
Long-Term Debt	$ 400,000,000						$ 400,000,000
Interest rate stated percentage (as a percentage)	2.375%						2.375%
NSTAR Electric Company | Unsecured Debt | 4.400% due 2044
Debt Instrument [Line Items]
Long-Term Debt	$ 300,000,000						$ 300,000,000
Interest rate stated percentage (as a percentage)	4.40%						4.40%
NSTAR Electric Company | Unsecured Debt | 3.250% due 2025
Debt Instrument [Line Items]
Long-Term Debt	$ 250,000,000						$ 250,000,000
Interest rate stated percentage (as a percentage)	3.25%						3.25%
NSTAR Electric Company | Unsecured Debt | 2.700% due 2026
Debt Instrument [Line Items]
Long-Term Debt	$ 250,000,000						$ 250,000,000
Interest rate stated percentage (as a percentage)	2.70%						2.70%
NSTAR Electric Company | Unsecured Debt | 3.200% due 2027
Debt Instrument [Line Items]
Long-Term Debt	$ 700,000,000						$ 0
Long-term debt	$ 700,000,000
Debt instrument, face amount			$ 350,000,000.0		$ 350,000,000.0
Interest rate stated percentage (as a percentage)	3.20%						3.20%
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]
Long-Term Debt	$ 50,000,000						$ 50,000,000
Interest rate stated percentage (as a percentage)	5.90%						5.90%
NSTAR Electric Company | Senior Notes | 6.700% Senior Notes Series D due 2037
Debt Instrument [Line Items]
Long-Term Debt	$ 40,000,000						$ 40,000,000
Interest rate stated percentage (as a percentage)	6.70%						6.70%
NSTAR Electric Company | Senior Notes | 5.100% Senior Notes Series E due 2020
Debt Instrument [Line Items]
Long-Term Debt	$ 95,000,000						$ 95,000,000
Interest rate stated percentage (as a percentage)	5.10%						5.10%
NSTAR Electric Company | Senior Notes | 3.500% Senior Notes Series F due 2021
Debt Instrument [Line Items]
Long-Term Debt	$ 250,000,000						$ 250,000,000
Interest rate stated percentage (as a percentage)	3.50%						3.50%
NSTAR Electric Company | Senior Notes | 3.880% Senior Notes Series G due 2023
Debt Instrument [Line Items]
Long-Term Debt	$ 80,000,000						$ 80,000,000
Interest rate stated percentage (as a percentage)	3.88%						3.88%
NSTAR Electric Company | Senior Notes | 2.750% Senior Notes Series H due 2026
Debt Instrument [Line Items]
Long-Term Debt	$ 50,000,000						$ 50,000,000
Interest rate stated percentage (as a percentage)	2.75%						2.75%
Public Service Company Of New Hampshire
Debt Instrument [Line Items]
Long-Term Debt	$ 892,438,000						$ 1,002,048,000
Less Amounts due Within One Year	(110,000,000)						(70,000,000)
Public Service Company Of New Hampshire | First Mortgage Notes
Debt Instrument [Line Items]
Long-Term Debt	917,000,000						987,000,000
Less Amounts due Within One Year	(110,000,000)						(70,000,000)
Unamortized Premiums and Discounts, Net	200,000						100,000
Unamortized Debt Issuance Costs	(4,100,000)						(4,400,000)
Long-term debt	892,400,000						1,002,000,000
Public Service Company Of New Hampshire | First Mortgage Notes | 5.600% Series M due 2035
Debt Instrument [Line Items]
Long-Term Debt	$ 50,000,000						$ 50,000,000
Interest rate stated percentage (as a percentage)	5.60%						5.60%
Public Service Company Of New Hampshire | First Mortgage Notes | 6.150% Series N due 2017
Debt Instrument [Line Items]
Long-Term Debt	$ 0						$ 70,000,000
Interest rate stated percentage (as a percentage)	6.15%						6.15%
Public Service Company Of New Hampshire | First Mortgage Notes | 6.000% Series O due 2018
Debt Instrument [Line Items]
Long-Term Debt	$ 110,000,000						$ 110,000,000
Interest rate stated percentage (as a percentage)	6.00%						6.00%
Public Service Company Of New Hampshire | First Mortgage Notes | 4.500% Series P due 2019
Debt Instrument [Line Items]
Long-Term Debt	$ 150,000,000						$ 150,000,000
Interest rate stated percentage (as a percentage)	4.50%						4.50%
Public Service Company Of New Hampshire | First Mortgage Notes | 4.050% Series Q due 2021
Debt Instrument [Line Items]
Long-Term Debt	$ 122,000,000						$ 122,000,000
Interest rate stated percentage (as a percentage)	4.05%						4.05%
Public Service Company Of New Hampshire | First Mortgage Notes | 3.200% Series R due 2021
Debt Instrument [Line Items]
Long-Term Debt	$ 160,000,000						$ 160,000,000
Interest rate stated percentage (as a percentage)	3.20%						3.20%
Public Service Company Of New Hampshire | First Mortgage Notes | 3.500% Series S due 2023
Debt Instrument [Line Items]
Long-Term Debt	$ 325,000,000						$ 325,000,000
Interest rate stated percentage (as a percentage)	3.50%						3.50%
Public Service Company Of New Hampshire | First Mortgage Notes | Adjustable Rate Tax Exempt Series A due 2021 (2.048% and 1.138% as of December 31, 2017 and 2016, respectively)
Debt Instrument [Line Items]
Long-Term Debt	$ 89,300,000						$ 89,300,000
Interest rate stated percentage (as a percentage)	1.138%						0.193%
Public Service Company Of New Hampshire | Senior Notes | 6.150% Series N due 2017
Debt Instrument [Line Items]
Interest rate stated percentage (as a percentage)	6.15%
Commercial Paper
Debt Instrument [Line Items]
Commercial Paper Classified as Long-Term Debt	$ 201,200,000.0						$ 0
Commercial Paper | NSTAR Electric Company
Debt Instrument [Line Items]
Commercial Paper Classified as Long-Term Debt	$ 650,000,000	$ 450,000,000
Minimum | First Mortgage Notes | NSTAR Gas - First Mortgage Bonds: 4.350% - 9.950% due 2020 - 2045
Debt Instrument [Line Items]
Interest rate stated percentage (as a percentage)	4.35%
Minimum | First Mortgage Notes | Yankee Gas - First Mortgage Bonds: 3.020% - 8.480% due 2018 - 2044
Debt Instrument [Line Items]
Interest rate stated percentage (as a percentage)	3.02%
Minimum | Unsecured Debt | 4.500% Debentures due 2019
Debt Instrument [Line Items]
Interest rate stated percentage (as a percentage)	4.50%
Minimum | Unsecured Debt | 1.450% - 4.000% Senior Notes due 2018 - 2026
Debt Instrument [Line Items]
Interest rate stated percentage (as a percentage)	1.45%
Minimum | Loans Payable | Notes Payable Unsecured 3.57% - 6.430% due 2021 - 2037
Debt Instrument [Line Items]
Interest rate stated percentage (as a percentage)	3.57%
Minimum | Loans Payable | Notes Payable Secured 4.10% - 9.64% due 2021 - 2035
Debt Instrument [Line Items]
Interest rate stated percentage (as a percentage)	4.10%
Maximum | NSTAR Gas - First Mortgage Bonds: 4.350% - 9.950% due 2020 - 2045
Debt Instrument [Line Items]
Interest rate stated percentage (as a percentage)							9.95%
Maximum | 1.450% - 4.000% Senior Notes due 2018 - 2026
Debt Instrument [Line Items]
Interest rate stated percentage (as a percentage)							4.00%
Maximum | Notes Payable Unsecured 3.57% - 6.430% due 2021 - 2037
Debt Instrument [Line Items]
Interest rate stated percentage (as a percentage)							6.43%
Maximum | Notes Payable Secured 4.10% - 9.64% due 2021 - 2035
Debt Instrument [Line Items]
Interest rate stated percentage (as a percentage)							9.64%
Maximum | First Mortgage Notes | Yankee Gas - First Mortgage Bonds: 3.020% - 8.480% due 2018 - 2044
Debt Instrument [Line Items]
Interest rate stated percentage (as a percentage)							8.48%
Maximum | Unsecured Debt | 4.500% Debentures due 2019
Debt Instrument [Line Items]
Interest rate stated percentage (as a percentage)							4.50%
Parent Company
Debt Instrument [Line Items]
Long-Term Debt	$ 3,523,472,000						$ 2,158,519,000
Less Amounts due Within One Year	(32,114,000)						$ (28,883,000)
Parent Company | Senior Notes | Eversource Parent 2.75% Series K Senior Notes
Debt Instrument [Line Items]
Long-term debt	$ 750,000,000
Debt instrument, face amount			$ 450,000,000.0			$ 300,000,000.0
Interest rate stated percentage (as a percentage)	2.75%
Parent Company | Commercial Paper
Debt Instrument [Line Items]
Commercial Paper Classified as Long-Term Debt	$ 1,450,000,000	$ 1,450,000,000.00