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LONG-TERM DEBT - Schedule of Long-Term Debt Outstanding (Details) - USD ($)
$ in Thousands
Jan. 31, 2025
Dec. 31, 2024
Dec. 31, 2023
Debt Instrument [Line Items]      
Long-Term Debt   $ 25,701,627 $ 23,588,616
Less Amounts due Within One Year   (1,003,150) (824,847)
Current Portion Classified as Long-Term Debt   0 990,900
Senior Notes      
Debt Instrument [Line Items]      
Unamortized Premiums and Discounts, Net     41,700 49,700
Unamortized Debt Issuance Costs   (76,100) (65,000)
Long-Term Debt   25,701,600 23,588,600
Pre-1983 Spent Nuclear Fuel Obligation (CYAPC)      
Debt Instrument [Line Items]      
Long-Term Debt   5,600 12,500
Fair Value Adjustment      
Debt Instrument [Line Items]      
Long-Term Debt   0 19,300
Fair Value Adjustment - Current Portion      
Debt Instrument [Line Items]      
Long-Term Debt   0 (5,500)
Loans Payable      
Debt Instrument [Line Items]      
Less Amounts due Within One Year   (750,300) (1,810,200)
Total Other Long-Term Debt      
Debt Instrument [Line Items]      
Long-Term Debt   14,016,400 12,845,700
Commercial Paper      
Debt Instrument [Line Items]      
Long-Term Debt     990,900
Eversource      
Debt Instrument [Line Items]      
Long-Term Debt   10,729,256 9,952,351
Less Amounts due Within One Year   (600,000) (364,653)
Eversource | Unsecured Debt | Eversource Parent - Senior Notes due 2025 - 2050      
Debt Instrument [Line Items]      
Long-Term Debt   $ 11,350,000 10,300,000
Eversource | Unsecured Debt | Eversource Parent - Senior Notes due 2025 - 2050 | Minimum      
Debt Instrument [Line Items]      
Interest rate   0.80%  
Eversource | Unsecured Debt | Eversource Parent - Senior Notes due 2025 - 2050 | Maximum      
Debt Instrument [Line Items]      
Interest rate   5.95%  
CL&P      
Debt Instrument [Line Items]      
Long-Term Debt   $ 5,108,173 4,814,429
Long-Term Debt, Due Within One Year   (400,000) (139,800)
Less Amounts due Within One Year   (2,944) 0
CL&P | Subsequent Event      
Debt Instrument [Line Items]      
Current Portion Classified as Long-Term Debt $ 397,100    
CL&P | First Mortgage Bonds      
Debt Instrument [Line Items]      
Long-Term Debt   5,130,000 4,619,800
Current Portion Classified as Long-Term Debt   397,100 139,800
Unamortized Premiums and Discounts, Net     14,300 18,000
Unamortized Debt Issuance Costs   (33,200) (30,700)
Long-Term Debt   $ 5,108,200 4,814,400
CL&P | First Mortgage Bonds | 1994 Series D due 2024      
Debt Instrument [Line Items]      
Interest rate   7.875%  
Long-Term Debt   $ 0 139,800
CL&P | First Mortgage Bonds | 2004 Series B due 2034      
Debt Instrument [Line Items]      
Interest rate   5.75%  
Long-Term Debt   $ 130,000 130,000
CL&P | First Mortgage Bonds | 2005 Series B due 2035      
Debt Instrument [Line Items]      
Interest rate   5.625%  
Long-Term Debt   $ 100,000 100,000
CL&P | First Mortgage Bonds | 2006 Series A due 2036      
Debt Instrument [Line Items]      
Interest rate   6.35%  
Long-Term Debt   $ 250,000 250,000
CL&P | First Mortgage Bonds | 2007 Series B due 2037      
Debt Instrument [Line Items]      
Interest rate   5.75%  
Long-Term Debt   $ 150,000 150,000
CL&P | First Mortgage Bonds | 2007 Series D due 2037      
Debt Instrument [Line Items]      
Interest rate   6.375%  
Long-Term Debt   $ 100,000 100,000
CL&P | First Mortgage Bonds | 2014 Series A due 2044        
Debt Instrument [Line Items]      
Interest rate   4.30%  
Long-Term Debt   $ 475,000 475,000
CL&P | First Mortgage Bonds | 2015 Series A due 2045      
Debt Instrument [Line Items]      
Interest rate   4.15%  
Long-Term Debt   $ 350,000 350,000
CL&P | First Mortgage Bonds | 2017 Series A due 2027      
Debt Instrument [Line Items]      
Interest rate   3.20%  
Long-Term Debt   $ 500,000 500,000
CL&P | First Mortgage Bonds | 2018 Series A due 2048      
Debt Instrument [Line Items]      
Interest rate   4.00%  
Long-Term Debt   $ 800,000 800,000
CL&P | First Mortgage Bonds | 2020 Series A due 2025      
Debt Instrument [Line Items]      
Interest rate   0.75%  
Long-Term Debt   $ 400,000 400,000
CL&P | First Mortgage Bonds | 2021 Series A due 2031      
Debt Instrument [Line Items]      
Interest rate   2.05%  
Long-Term Debt   $ 425,000 425,000
CL&P | First Mortgage Bonds | 2023 Series A due 2053      
Debt Instrument [Line Items]      
Interest rate   5.25%  
Long-Term Debt   $ 500,000 500,000
CL&P | First Mortgage Bonds | 2023 Series B due 2033      
Debt Instrument [Line Items]      
Interest rate   4.90%  
Long-Term Debt   $ 300,000 300,000
CL&P | First Mortgage Bonds | 2024 Series A due 2029      
Debt Instrument [Line Items]      
Interest rate   4.65%  
Long-Term Debt   $ 350,000 0
CL&P | First Mortgage Bonds | 2024 Series B due 2034      
Debt Instrument [Line Items]      
Interest rate   4.95%  
Long-Term Debt   $ 300,000 0
CL&P | Commercial Paper      
Debt Instrument [Line Items]      
Long-Term Debt     139,800
Current Portion Classified as Long-Term Debt   0 207,300
NSTAR Electric      
Debt Instrument [Line Items]      
Long-Term Debt   4,844,920 4,496,947
Less Amounts due Within One Year   (250,000) 0
NSTAR Electric | Unsecured Debt      
Debt Instrument [Line Items]      
Long-Term Debt   5,000,000 4,400,000
Less Amounts due Within One Year   (250,000) 0
Unamortized Premiums and Discounts, Net     (14,000) (14,000)
Unamortized Debt Issuance Costs   (31,100) (29,100)
Long-Term Debt   $ 4,844,900 4,496,900
NSTAR Electric | Unsecured Debt | 2006 Debentures due 2036      
Debt Instrument [Line Items]      
Interest rate   5.75%  
Long-Term Debt   $ 200,000 200,000
NSTAR Electric | Unsecured Debt | 2010 Debentures due 2040      
Debt Instrument [Line Items]      
Interest rate   5.50%  
Long-Term Debt   $ 300,000 300,000
NSTAR Electric | Unsecured Debt | 2014 Debentures due 2044        
Debt Instrument [Line Items]      
Interest rate   4.40%  
Long-Term Debt   $ 300,000 300,000
NSTAR Electric | Unsecured Debt | 2015 Debentures due 2025      
Debt Instrument [Line Items]      
Interest rate   3.25%  
Long-Term Debt   $ 250,000 250,000
NSTAR Electric | Unsecured Debt | 2016 Debentures due 2026      
Debt Instrument [Line Items]      
Interest rate   2.70%  
Long-Term Debt   $ 250,000 250,000
NSTAR Electric | Unsecured Debt | 2017 Debentures due 2027      
Debt Instrument [Line Items]      
Interest rate   3.20%  
Long-Term Debt   $ 700,000 700,000
NSTAR Electric | Unsecured Debt | 2019 Debentures due 2029      
Debt Instrument [Line Items]      
Interest rate   3.25%  
Long-Term Debt   $ 400,000 400,000
NSTAR Electric | Unsecured Debt | 2020 Debentures due 2030      
Debt Instrument [Line Items]      
Interest rate   3.95%  
Long-Term Debt   $ 400,000 400,000
NSTAR Electric | Unsecured Debt | 2021 Debentures due 2051      
Debt Instrument [Line Items]      
Interest rate   3.10%  
Long-Term Debt   $ 300,000 300,000
NSTAR Electric | Unsecured Debt | 2021 Debentures due 2031      
Debt Instrument [Line Items]      
Interest rate   1.95%  
Long-Term Debt   $ 300,000 300,000
NSTAR Electric | Unsecured Debt | 2022 Debentures due 2052      
Debt Instrument [Line Items]      
Interest rate   4.55%  
Long-Term Debt   $ 450,000 450,000
NSTAR Electric | Unsecured Debt | 2022 Debentures due 2052      
Debt Instrument [Line Items]      
Interest rate   4.95%  
Long-Term Debt   $ 400,000 400,000
NSTAR Electric | Unsecured Debt | 2023 Debentures due 2028      
Debt Instrument [Line Items]      
Interest rate   5.60%  
Long-Term Debt   $ 150,000 150,000
NSTAR Electric | Unsecured Debt | 2024 Debentures due 2034      
Debt Instrument [Line Items]      
Interest rate   5.40%  
Long-Term Debt   $ 600,000 0
NSTAR Electric | Senior Notes      
Debt Instrument [Line Items]      
Long-Term Debt   $ 140,000 140,000
NSTAR Electric | Senior Notes | 2004 Senior Notes Series B due 2034      
Debt Instrument [Line Items]      
Interest rate   5.90%  
Long-Term Debt   $ 50,000 50,000
NSTAR Electric | Senior Notes | 2007 Senior Notes Series D due 2037      
Debt Instrument [Line Items]      
Interest rate   6.70%  
Long-Term Debt   $ 40,000 40,000
NSTAR Electric | Senior Notes | 2016 Senior Notes Series H due 2026      
Debt Instrument [Line Items]      
Interest rate   2.75%  
Long-Term Debt   $ 50,000 50,000
PSNH      
Debt Instrument [Line Items]      
Long-Term Debt   1,732,066 1,431,591
PSNH | First Mortgage Bonds      
Debt Instrument [Line Items]      
Long-Term Debt   1,750,000 1,450,000
Unamortized Premiums and Discounts, Net     (2,600) (4,900)
Unamortized Debt Issuance Costs   (15,300) (13,500)
Long-Term Debt   $ 1,732,100 1,431,600
PSNH | First Mortgage Bonds | 2005 Series M due 2035      
Debt Instrument [Line Items]      
Interest rate   5.60%  
Long-Term Debt   $ 50,000 50,000
PSNH | First Mortgage Bonds | 2019 Series T due 2049        
Debt Instrument [Line Items]      
Interest rate   3.60%  
Long-Term Debt   $ 300,000 300,000
PSNH | First Mortgage Bonds | 2020 Series U due 2050      
Debt Instrument [Line Items]      
Interest rate   2.40%  
Long-Term Debt   $ 150,000 150,000
PSNH | First Mortgage Bonds | 2021 Series V due 2031      
Debt Instrument [Line Items]      
Interest rate   2.20%  
Long-Term Debt   $ 350,000 350,000
PSNH | First Mortgage Bonds | 2023 Series W due 2053      
Debt Instrument [Line Items]      
Interest rate   5.15%  
Long-Term Debt   $ 300,000 300,000
PSNH | First Mortgage Bonds | 2023 Series X due 2033      
Debt Instrument [Line Items]      
Interest rate   5.35%  
Long-Term Debt   $ 600,000 300,000
Yankee Gas | First Mortgage Bonds | Yankee Gas - First Mortgage Bonds due 2025 - 2051      
Debt Instrument [Line Items]      
Long-Term Debt   $ 1,095,000 1,015,000
Yankee Gas | First Mortgage Bonds | Yankee Gas - First Mortgage Bonds due 2025 - 2051 | Minimum      
Debt Instrument [Line Items]      
Interest rate   1.38%  
Yankee Gas | First Mortgage Bonds | Yankee Gas - First Mortgage Bonds due 2025 - 2051 | Maximum      
Debt Instrument [Line Items]      
Interest rate   5.74%  
NSTAR Gas | First Mortgage Bonds | NSTAR Gas - First Mortgage Bonds due 2025 - 2051      
Debt Instrument [Line Items]      
Long-Term Debt   $ 905,000 705,000
NSTAR Gas | First Mortgage Bonds | NSTAR Gas - First Mortgage Bonds due 2025 - 2051 | Minimum      
Debt Instrument [Line Items]      
Interest rate   2.25%  
NSTAR Gas | First Mortgage Bonds | NSTAR Gas - First Mortgage Bonds due 2025 - 2051 | Maximum      
Debt Instrument [Line Items]      
Interest rate   7.11%  
EGMA | First Mortgage Bonds | Aquarion - Unsecured Notes due 2028 - 2052      
Debt Instrument [Line Items]      
Long-Term Debt   $ 808,000 708,000
EGMA | First Mortgage Bonds | Aquarion - Unsecured Notes due 2028 - 2052 | Minimum      
Debt Instrument [Line Items]      
Interest rate   2.11%  
EGMA | First Mortgage Bonds | Aquarion - Unsecured Notes due 2028 - 2052 | Maximum      
Debt Instrument [Line Items]      
Interest rate   5.73%  
Aquarion | Unsecured Debt | Aquarion - Unsecured Notes due 2028 - 2052      
Debt Instrument [Line Items]      
Long-Term Debt   $ 596,800 527,000
Aquarion | Unsecured Debt | Aquarion - Unsecured Notes due 2028 - 2052 | Minimum      
Debt Instrument [Line Items]      
Interest rate   3.00%  
Aquarion | Unsecured Debt | Aquarion - Unsecured Notes due 2028 - 2052 | Maximum      
Debt Instrument [Line Items]      
Interest rate   6.43%  
Aquarion | Secured Debt | Aquarion - Secured Debt due 2027 - 2045      
Debt Instrument [Line Items]      
Long-Term Debt   $ 40,700 39,000
Aquarion | Secured Debt | Aquarion - Secured Debt due 2027 - 2045 | Minimum      
Debt Instrument [Line Items]      
Interest rate   1.296%  
Aquarion | Secured Debt | Aquarion - Secured Debt due 2027 - 2045 | Maximum      
Debt Instrument [Line Items]      
Interest rate   9.29%  
Aquarion | Senior Notes | Aquarion - Senior Notes due 2024      
Debt Instrument [Line Items]      
Interest rate   4.00%  
Long-Term Debt   $ 0 $ 360,000