LONG-TERM DEBT - Schedule of Long-Term Debt Outstanding (Details) - USD ($) $ in Thousands	Dec. 31, 2020	Dec. 31, 2019
Debt Instrument [Line Items]
Long-Term Debt	$ 15,125,876	$ 13,770,828
Less Amounts due in One Year	(1,053,186)	(327,411)
Commercial Paper Classified as Long-Term Debt	0	346,300
Senior Notes
Debt Instrument [Line Items]
Unamortized Premiums and Discounts, Net	46,500	(4,100)
Unamortized Debt Issuance Costs	(32,000)	(20,600)
Long-Term Debt	15,125,900	13,770,800
Pre-1983 Spent Nuclear Fuel Obligation (CYAPC)
Debt Instrument [Line Items]
Long-Term Debt	11,700	11,600
Fair Value Adjustment
Debt Instrument [Line Items]
Long-Term Debt	74,700	109,100
Fair Value Adjustment - Current Portion
Debt Instrument [Line Items]
Long-Term Debt	(31,000)	(31,300)
Loans Payable
Debt Instrument [Line Items]
Less Amounts due in One Year	(490,200)	(201,100)
Total Other Long-Term Debt
Debt Instrument [Line Items]
Long-Term Debt	7,000,800	6,054,000
The Connecticut Light and Power Company
Debt Instrument [Line Items]
Long-Term Debt	3,914,835	3,518,136
Less Amounts due in One Year	$ 0	0
The Connecticut Light and Power Company | 4.375% Fixed Rate Tax Exempt due 2028
Debt Instrument [Line Items]
Interest rate	4.375%
Long-Term Debt	$ 120,500	120,500
The Connecticut Light and Power Company | First Mortgage Bonds
Debt Instrument [Line Items]
Long-Term Debt	3,794,800	3,394,800
Unamortized Premiums and Discounts, Net	25,900	27,800
Unamortized Debt Issuance Costs	(26,400)	(25,000)
Long-Term Debt	$ 3,914,800	3,518,100
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	0
NSTAR Electric Company
Debt Instrument [Line Items]
Long-Term Debt	3,393,221	3,247,086
Less Amounts due in One Year	(250,000)	(95,000)
NSTAR Electric Company | Unsecured Debt
Debt Instrument [Line Items]
Long-Term Debt	3,200,000	2,800,000
Less Amounts due in One Year	(250,000)	(95,000)
Unamortized Premiums and Discounts, Net	(6,800)	(4,100)
Unamortized Debt Issuance Costs	(20,000)	(18,800)
Long-Term Debt	$ 3,393,200	3,247,100
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	0
NSTAR Electric Company | Senior Notes
Debt Instrument [Line Items]
Long-Term Debt	$ 470,000	565,000
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 | 5.100% Senior Notes Series E due 2020
Debt Instrument [Line Items]
Long-Term Debt	$ 0	95,000
NSTAR Electric Company | Senior Notes | 3.500% Senior Notes Series F due 2021
Debt Instrument [Line Items]
Interest rate	3.50%
Long-Term Debt	$ 250,000	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	817,070	951,620
Less Amounts due in One Year	(282,000)	0
Public Service Company of New Hampshire | First Mortgage Bonds
Debt Instrument [Line Items]
Long-Term Debt	1,107,000	957,000
Less Amounts due in One Year	(282,000)	0
Unamortized Premiums and Discounts, Net	(1,500)	(700)
Unamortized Debt Issuance Costs	(6,400)	(4,700)
Long-Term Debt	$ 817,100	951,600
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	$ 122,000	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	$ 160,000	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]
Long-Term Debt	150,000	0
Yankee Gas | First Mortgage Bonds | Yankee Gas - First Mortgage Bonds: 2.230% - 8.480% due 2022 - 2050
Debt Instrument [Line Items]
Long-Term Debt	$ 640,000	620,000
Yankee Gas | First Mortgage Bonds | Yankee Gas - First Mortgage Bonds: 2.230% - 8.480% due 2022 - 2050 | Minimum
Debt Instrument [Line Items]
Interest rate	2.23%
Yankee Gas | First Mortgage Bonds | Yankee Gas - First Mortgage Bonds: 2.230% - 8.480% due 2022 - 2050 | Maximum
Debt Instrument [Line Items]
Interest rate	8.48%
NSTAR Gas | First Mortgage Bonds | NSTAR Gas - First Mortgage Bonds: 2.330% - 7.110% due 2025 - 2050
Debt Instrument [Line Items]
Long-Term Debt	$ 500,000	460,000
NSTAR Gas | First Mortgage Bonds | NSTAR Gas - First Mortgage Bonds: 2.330% - 7.110% due 2025 - 2050 | Minimum
Debt Instrument [Line Items]
Interest rate	2.33%
NSTAR Gas | First Mortgage Bonds | NSTAR Gas - First Mortgage Bonds: 2.330% - 7.110% due 2025 - 2050 | Maximum
Debt Instrument [Line Items]
Interest rate	7.11%
Aquarion | Unsecured Debt | Aquarion - Senior Note 4.000% due 2024
Debt Instrument [Line Items]
Interest rate	4.00%
Long-Term Debt	$ 360,000	360,000
Aquarion | Unsecured Debt | Aquarion - Unsecured Notes 0% - 6.430% due 2021 - 2049
Debt Instrument [Line Items]
Long-Term Debt	$ 335,200	335,300
Aquarion | Unsecured Debt | Aquarion - Unsecured Notes 0% - 6.430% due 2021 - 2049 | Minimum
Debt Instrument [Line Items]
Interest rate	0.00%
Aquarion | Unsecured Debt | Aquarion - Unsecured Notes 0% - 6.430% due 2021 - 2049 | Maximum
Debt Instrument [Line Items]
Interest rate	6.43%
Aquarion | Secured Debt | Aquarion - Secured Debt 4.450% - 9.290% due 2022 - 2035
Debt Instrument [Line Items]
Long-Term Debt	$ 35,900	68,800
Aquarion | Secured Debt | Aquarion - Secured Debt 4.450% - 9.290% due 2022 - 2035 | Minimum
Debt Instrument [Line Items]
Interest rate	4.45%
Aquarion | Secured Debt | Aquarion - Secured Debt 4.450% - 9.290% due 2022 - 2035 | Maximum
Debt Instrument [Line Items]
Interest rate	9.29%
Eversource
Debt Instrument [Line Items]
Long-Term Debt	$ 5,128,322	4,358,339
Less Amounts due in One Year	(473,933)	(23,933)
Eversource | Unsecured Debt | Eversource Parent - Senior Notes 0.800% - 4.250% due 2021 - 2050
Debt Instrument [Line Items]
Long-Term Debt	$ 5,550,000	$ 4,000,000
Eversource | Unsecured Debt | Eversource Parent - Senior Notes 0.800% - 4.250% due 2021 - 2050 | Minimum
Debt Instrument [Line Items]
Interest rate	0.80%
Eversource | Unsecured Debt | Eversource Parent - Senior Notes 0.800% - 4.250% due 2021 - 2050 | Maximum
Debt Instrument [Line Items]
Interest rate	4.25%