LONG-TERM DEBT - Schedule of Long-Term Debt Outstanding (Details) - USD ($) $ in Thousands	Dec. 31, 2021	Oct. 31, 2021	Aug. 31, 2021	Jun. 30, 2021	May 31, 2021	Mar. 31, 2021	Dec. 31, 2020	Aug. 31, 2020
Debt Instrument [Line Items]
Long-Term Debt	$ 17,023,577						$ 15,125,876
Less Amounts due in One Year	(1,193,097)						(1,053,186)
Senior Notes
Debt Instrument [Line Items]
Unamortized Premiums and Discounts, Net	43,400						46,500
Unamortized Debt Issuance Costs	(36,300)						(32,000)
Long-Term Debt	17,023,600						15,125,900
Pre-1983 Spent Nuclear Fuel Obligation (CYAPC)
Debt Instrument [Line Items]
Long-Term Debt	11,700						11,700
Fair Value Adjustment
Debt Instrument [Line Items]
Long-Term Debt	43,800						74,700
Fair Value Adjustment - Current Portion
Debt Instrument [Line Items]
Long-Term Debt	(17,700)						(31,000)
Loans Payable
Debt Instrument [Line Items]
Less Amounts due in One Year	(775,400)						(490,200)
Total Other Long-Term Debt
Debt Instrument [Line Items]
Long-Term Debt	8,059,000						7,000,800
Eversource
Debt Instrument [Line Items]
Long-Term Debt	5,395,840						5,128,322
Less Amounts due in One Year	(767,681)						(473,933)
Eversource | Unsecured Debt | Eversource Parent - Senior Notes 0.300% - 4.250% due 2022 - 2050
Debt Instrument [Line Items]
Long-Term Debt	$ 6,100,000						5,550,000
Eversource | Unsecured Debt | Eversource Parent - Senior Notes 0.300% - 4.250% due 2022 - 2050 | Minimum
Debt Instrument [Line Items]
Interest rate	0.30%
Eversource | Unsecured Debt | Eversource Parent - Senior Notes 0.300% - 4.250% due 2022 - 2050 | Maximum
Debt Instrument [Line Items]
Interest rate	4.25%
The Connecticut Light and Power Company
Debt Instrument [Line Items]
Long-Term Debt	$ 4,215,379						3,914,835
The Connecticut Light and Power Company | 4.375% Fixed Rate Tax Exempt due 2028
Debt Instrument [Line Items]
Long-Term Debt	0						120,500
The Connecticut Light and Power Company | First Mortgage Bonds
Debt Instrument [Line Items]
Long-Term Debt	4,219,800						3,794,800
Unamortized Premiums and Discounts, Net	23,100						25,900
Unamortized Debt Issuance Costs	(27,500)						(26,400)
Long-Term Debt	$ 4,215,400						3,914,800
The Connecticut Light and Power Company | First Mortgage Bonds | 7.875% 1994 Series D due 2024
Debt Instrument [Line Items]
Interest rate	7.875%
Long-Term Debt	$ 139,800						139,800
The Connecticut Light and Power Company | First Mortgage Bonds | 5.750% 2004 Series B due 2034
Debt Instrument [Line Items]
Interest rate	5.75%
Long-Term Debt	$ 130,000						130,000
The Connecticut Light and Power Company | First Mortgage Bonds | 5.625% 2005 Series B due 2035
Debt Instrument [Line Items]
Interest rate	5.625%
Long-Term Debt	$ 100,000						100,000
The Connecticut Light and Power Company | First Mortgage Bonds | 6.350% 2006 Series A due 2036
Debt Instrument [Line Items]
Interest rate	6.35%
Long-Term Debt	$ 250,000						250,000
The Connecticut Light and Power Company | First Mortgage Bonds | 5.750% 2007 Series B due 2037
Debt Instrument [Line Items]
Interest rate	5.75%
Long-Term Debt	$ 150,000						150,000
The Connecticut Light and Power Company | First Mortgage Bonds | 6.375% 2007 Series D due 2037
Debt Instrument [Line Items]
Interest rate	6.375%
Long-Term Debt	$ 100,000						100,000
The Connecticut Light and Power Company | First Mortgage Bonds | 2.500% 2013 Series A due 2023
Debt Instrument [Line Items]
Interest rate	2.50%
Long-Term Debt	$ 400,000						400,000
The Connecticut Light and Power Company | First Mortgage Bonds | 4.300% 2014 Series A due 2044
Debt Instrument [Line Items]
Interest rate	4.30%
Long-Term Debt	$ 475,000						475,000
The Connecticut Light and Power Company | First Mortgage Bonds | 4.150% 2015 Series A due 2045
Debt Instrument [Line Items]
Interest rate	4.15%
Long-Term Debt	$ 350,000						350,000
The Connecticut Light and Power Company | First Mortgage Bonds | 3.200% 2017 Series A due 2027
Debt Instrument [Line Items]
Interest rate	3.20%
Long-Term Debt	$ 500,000						500,000
The Connecticut Light and Power Company | First Mortgage Bonds | 4.000% 2018 Series A due 2048
Debt Instrument [Line Items]
Interest rate	4.00%
Long-Term Debt	$ 800,000						800,000
The Connecticut Light and Power Company | First Mortgage Bonds | 0.750% 2020 Series A due 2025
Debt Instrument [Line Items]
Interest rate	0.75%
Long-Term Debt	$ 400,000						400,000
The Connecticut Light and Power Company | First Mortgage Bonds | 2.050% 2021 Series A due 2031
Debt Instrument [Line Items]
Interest rate	2.05%
Long-Term Debt	$ 425,000						0
NSTAR Electric Company
Debt Instrument [Line Items]
Long-Term Debt	3,585,399						3,393,221
Less Amounts due in One Year	(400,000)						(250,000)
NSTAR Electric Company | Unsecured Debt
Debt Instrument [Line Items]
Long-Term Debt	3,800,000						3,200,000
Less Amounts due in One Year	(400,000)						(250,000)
Unamortized Premiums and Discounts, Net	(11,200)						(6,800)
Unamortized Debt Issuance Costs	(23,400)						(20,000)
Long-Term Debt	$ 3,585,400						3,393,200
NSTAR Electric Company | Unsecured Debt | 5.750% due 2036
Debt Instrument [Line Items]
Interest rate	5.75%
Long-Term Debt	$ 200,000						200,000
NSTAR Electric Company | Unsecured Debt | 5.500% due 2040
Debt Instrument [Line Items]
Interest rate	5.50%
Long-Term Debt	$ 300,000						300,000
NSTAR Electric Company | Unsecured Debt | 2.375% due 2022
Debt Instrument [Line Items]
Interest rate	2.375%
Long-Term Debt	$ 400,000						400,000
NSTAR Electric Company | Unsecured Debt | 4.400% due 2044
Debt Instrument [Line Items]
Interest rate	4.40%
Long-Term Debt	$ 300,000						300,000
NSTAR Electric Company | Unsecured Debt | 3.250% due 2025
Debt Instrument [Line Items]
Interest rate	3.25%
Long-Term Debt	$ 250,000						250,000
NSTAR Electric Company | Unsecured Debt | 2.700% due 2026
Debt Instrument [Line Items]
Interest rate	2.70%
Long-Term Debt	$ 250,000						250,000
NSTAR Electric Company | Unsecured Debt | 3.200% due 2027
Debt Instrument [Line Items]
Interest rate	3.20%
Long-Term Debt	$ 700,000						700,000
NSTAR Electric Company | Unsecured Debt | 3.250% due 2029
Debt Instrument [Line Items]
Interest rate	3.25%
Long-Term Debt	$ 400,000						400,000
NSTAR Electric Company | Unsecured Debt | 3.950% due 2030
Debt Instrument [Line Items]
Interest rate	3.95%
Long-Term Debt	$ 400,000						400,000
NSTAR Electric Company | Unsecured Debt | 3.100% due 2051
Debt Instrument [Line Items]
Interest rate					3.10%
Long-Term Debt	300,000						0
NSTAR Electric Company | Unsecured Debt | 1.950% due 2031
Debt Instrument [Line Items]
Interest rate			1.95%
Long-Term Debt	300,000						0
NSTAR Electric Company | Senior Notes
Debt Instrument [Line Items]
Long-Term Debt	$ 220,000						470,000
NSTAR Electric Company | Senior Notes | 5.900% Senior Notes Series B due 2034
Debt Instrument [Line Items]
Interest rate	5.90%
Long-Term Debt	$ 50,000						50,000
NSTAR Electric Company | Senior Notes | 6.700% Senior Notes Series D due 2037
Debt Instrument [Line Items]
Interest rate	6.70%
Long-Term Debt	$ 40,000						40,000
NSTAR Electric Company | Senior Notes | 3.500% Senior Notes Series F due 2021
Debt Instrument [Line Items]
Interest rate				3.50%
Long-Term Debt	$ 0						250,000
NSTAR Electric Company | Senior Notes | 3.880% Senior Notes Series G due 2023
Debt Instrument [Line Items]
Interest rate	3.88%
Long-Term Debt	$ 80,000						80,000
NSTAR Electric Company | Senior Notes | 2.750% Senior Notes Series H due 2026
Debt Instrument [Line Items]
Interest rate	2.75%
Long-Term Debt	$ 50,000						50,000
Public Service Company of New Hampshire
Debt Instrument [Line Items]
Long-Term Debt	1,163,833						817,070
Less Amounts due in One Year	0						(282,000)
Public Service Company of New Hampshire | First Mortgage Bonds
Debt Instrument [Line Items]
Long-Term Debt	1,175,000						1,107,000
Less Amounts due in One Year	0						(282,000)
Unamortized Premiums and Discounts, Net	(2,600)						(1,500)
Unamortized Debt Issuance Costs	(8,600)						(6,400)
Long-Term Debt	$ 1,163,800						817,100
Public Service Company of New Hampshire | First Mortgage Bonds | 5.600% Series M due 2035
Debt Instrument [Line Items]
Interest rate	5.60%
Long-Term Debt	$ 50,000						50,000
Public Service Company of New Hampshire | First Mortgage Bonds | 4.050% Series Q due 2021
Debt Instrument [Line Items]
Interest rate						4.05%
Long-Term Debt	0						122,000
Public Service Company of New Hampshire | First Mortgage Bonds | 3.200% Series R due 2021
Debt Instrument [Line Items]
Interest rate				3.20%
Long-Term Debt	$ 0						160,000
Public Service Company of New Hampshire | First Mortgage Bonds | 3.500% Series S due 2023
Debt Instrument [Line Items]
Interest rate	3.50%
Long-Term Debt	$ 325,000						325,000
Public Service Company of New Hampshire | First Mortgage Bonds | 3.600% Series T due 2049
Debt Instrument [Line Items]
Interest rate	3.60%
Long-Term Debt	$ 300,000						300,000
Public Service Company of New Hampshire | First Mortgage Bonds | 2.400% Series U due 2050
Debt Instrument [Line Items]
Interest rate								2.40%
Long-Term Debt	150,000						150,000
Public Service Company of New Hampshire | First Mortgage Bonds | 2.200% Series V due 2031
Debt Instrument [Line Items]
Interest rate				2.20%
Long-Term Debt	350,000						0
Yankee Gas | First Mortgage Bonds | Yankee Gas - First Mortgage Bonds: 1.380% - 8.480% due 2022 - 2051
Debt Instrument [Line Items]
Long-Term Debt	$ 765,000						640,000
Yankee Gas | First Mortgage Bonds | Yankee Gas - First Mortgage Bonds: 1.380% - 8.480% due 2022 - 2051 | Minimum
Debt Instrument [Line Items]
Interest rate	1.38%
Yankee Gas | First Mortgage Bonds | Yankee Gas - First Mortgage Bonds: 1.380% - 8.480% due 2022 - 2051 | Maximum
Debt Instrument [Line Items]
Interest rate	8.48%
NSTAR Gas | First Mortgage Bonds | NSTAR Gas - First Mortgage Bonds: 2.250% - 7.110% due 2025 - 2051
Debt Instrument [Line Items]
Long-Term Debt	$ 580,000						500,000
NSTAR Gas | First Mortgage Bonds | NSTAR Gas - First Mortgage Bonds: 2.250% - 7.110% due 2025 - 2051 | Minimum
Debt Instrument [Line Items]
Interest rate		2.25%
NSTAR Gas | First Mortgage Bonds | NSTAR Gas - First Mortgage Bonds: 2.250% - 7.110% due 2025 - 2051 | Maximum
Debt Instrument [Line Items]
Interest rate	7.11%
EGMA | First Mortgage Bonds | EGMA - First Mortgage Bonds: 2.110% - 2.920% due 2031 - 2051
Debt Instrument [Line Items]
Long-Term Debt	$ 550,000						0
EGMA | First Mortgage Bonds | EGMA - First Mortgage Bonds: 2.110% - 2.920% due 2031 - 2051 | Minimum
Debt Instrument [Line Items]
Interest rate	2.11%
EGMA | First Mortgage Bonds | EGMA - First Mortgage Bonds: 2.110% - 2.920% due 2031 - 2051 | Maximum
Debt Instrument [Line Items]
Interest rate	2.92%
Aquarion | Unsecured Debt | Aquarion - Senior Notes 4.000% due 2024
Debt Instrument [Line Items]
Long-Term Debt	$ 360,000						360,000
Aquarion | Unsecured Debt | Aquarion - Senior Notes 4.000% due 2024 | Maximum
Debt Instrument [Line Items]
Interest rate	4.00%
Aquarion | Unsecured Debt | Aquarion - Unsecured Notes 0% - 6.430% due 2023 - 2051
Debt Instrument [Line Items]
Long-Term Debt	$ 394,900						335,200
Aquarion | Unsecured Debt | Aquarion - Unsecured Notes 0% - 6.430% due 2023 - 2051 | Minimum
Debt Instrument [Line Items]
Interest rate	0.00%
Aquarion | Unsecured Debt | Aquarion - Unsecured Notes 0% - 6.430% due 2023 - 2051 | Maximum
Debt Instrument [Line Items]
Interest rate	6.43%
Aquarion | Secured Debt | Aquarion - Secured Debt 1.296% - 9.290% due 2022 - 2044
Debt Instrument [Line Items]
Long-Term Debt	$ 39,600						$ 35,900
Aquarion | Secured Debt | Aquarion - Secured Debt 1.296% - 9.290% due 2022 - 2044 | Minimum
Debt Instrument [Line Items]
Interest rate	1.296%
Aquarion | Secured Debt | Aquarion - Secured Debt 1.296% - 9.290% due 2022 - 2044 | Maximum
Debt Instrument [Line Items]
Interest rate	9.29%