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REGULATORY ACCOUNTING - Components of Regulatory Assets (Details) - USD ($) $ in Thousands | Sep. 30, 2021 | Dec. 31, 2020
--- | --- | ---
Regulatory Assets [Line Items] |  |
Total Regulatory Assets | $ 6,475,200 | $ 6,569,900
Less: Current Portion | 1,016,304 | 1,076,556
Total Long-Term Regulatory Assets | 5,458,928 | 5,493,330
Benefit Costs |  |
Regulatory Assets [Line Items] |  |
Total Regulatory Assets | 2,566,100 | 2,794,200
Income Taxes, Net |  |
Regulatory Assets [Line Items] |  |
Total Regulatory Assets | 756,400 | 747,100
Securitized Stranded Costs |  |
Regulatory Assets [Line Items] |  |
Total Regulatory Assets | 489,700 | 522,100
Storm Restoration Costs, Net |  |
Regulatory Assets [Line Items] |  |
Total Regulatory Assets | 944,200 | 765,600
Regulatory Tracker Mechanisms |  |
Regulatory Assets [Line Items] |  |
Total Regulatory Assets | 883,000 | 850,500
Derivative Liabilities |  |
Regulatory Assets [Line Items] |  |
Total Regulatory Assets | 265,300 | 296,300
Goodwill-related |  |
Regulatory Assets [Line Items] |  |
Total Regulatory Assets | 302,000 | 314,700
Asset Retirement Obligations |  |
Regulatory Assets [Line Items] |  |
Total Regulatory Assets | 121,600 | 118,400
Other Regulatory Assets |  |
Regulatory Assets [Line Items] |  |
Total Regulatory Assets | 146,900 | 161,000
CL&P |  |
Regulatory Assets [Line Items] |  |
Total Regulatory Assets | 2,307,100 | 2,211,800
Less: Current Portion | 325,268 | 345,622
Total Long-Term Regulatory Assets | 1,981,805 | 1,866,152
CL&P | Benefit Costs |  |
Regulatory Assets [Line Items] |  |
Total Regulatory Assets | 540,200 | 632,300
CL&P | Income Taxes, Net |  |
Regulatory Assets [Line Items] |  |
Total Regulatory Assets | 458,100 | 458,900
CL&P | Securitized Stranded Costs |  |
Regulatory Assets [Line Items] |  |
Total Regulatory Assets | 0 | 0
CL&P | Storm Restoration Costs, Net |  |
Regulatory Assets [Line Items] |  |
Total Regulatory Assets | 674,200 | 515,100
CL&P | Regulatory Tracker Mechanisms |  |
Regulatory Assets [Line Items] |  |
Total Regulatory Assets | 304,600 | 246,600
CL&P | Derivative Liabilities |  |
Regulatory Assets [Line Items] |  |
Total Regulatory Assets | 265,300 | 293,100
CL&P | Goodwill-related |  |
Regulatory Assets [Line Items] |  |
Total Regulatory Assets | 0 | 0
CL&P | Asset Retirement Obligations |  |
Regulatory Assets [Line Items] |  |
Total Regulatory Assets | 33,700 | 32,100
CL&P | Other Regulatory Assets |  |
Regulatory Assets [Line Items] |  |
Total Regulatory Assets | 31,000 | 33,700
NSTAR Electric |  |
Regulatory Assets [Line Items] |  |
Total Regulatory Assets | 1,636,100 | 1,703,900
Less: Current Portion | 388,408 | 399,882
Total Long-Term Regulatory Assets | 1,247,715 | 1,304,019
NSTAR Electric | Benefit Costs |  |
Regulatory Assets [Line Items] |  |
Total Regulatory Assets | 638,400 | 690,000
NSTAR Electric | Income Taxes, Net |  |
Regulatory Assets [Line Items] |  |
Total Regulatory Assets | 113,800 | 110,400
NSTAR Electric | Securitized Stranded Costs |  |
Regulatory Assets [Line Items] |  |
Total Regulatory Assets | 0 | 0
NSTAR Electric | Storm Restoration Costs, Net |  |
Regulatory Assets [Line Items] |  |
Total Regulatory Assets | 198,100 | 186,400
NSTAR Electric | Regulatory Tracker Mechanisms |  |
Regulatory Assets [Line Items] |  |
Total Regulatory Assets | 320,000 | 332,200
NSTAR Electric | Derivative Liabilities |  |
Regulatory Assets [Line Items] |  |
Total Regulatory Assets | 0 | 0
NSTAR Electric | Goodwill-related |  |
Regulatory Assets [Line Items] |  |
Total Regulatory Assets | 259,300 | 270,200
NSTAR Electric | Asset Retirement Obligations |  |
Regulatory Assets [Line Items] |  |
Total Regulatory Assets | 57,800 | 58,600
NSTAR Electric | Other Regulatory Assets |  |
Regulatory Assets [Line Items] |  |
Total Regulatory Assets | 48,700 | 56,100
PSNH |  |
Regulatory Assets [Line Items] |  |
Total Regulatory Assets | 927,700 | 989,100
Less: Current Portion | 97,948 | 115,852
Total Long-Term Regulatory Assets | 829,848 | 873,203
PSNH | Benefit Costs |  |
Regulatory Assets [Line Items] |  |
Total Regulatory Assets | 249,700 | 267,600
PSNH | Income Taxes, Net |  |
Regulatory Assets [Line Items] |  |
Total Regulatory Assets | 18,700 | 15,200
PSNH | Securitized Stranded Costs |  |
Regulatory Assets [Line Items] |  |
Total Regulatory Assets | 489,700 | 522,100
PSNH | Storm Restoration Costs, Net |  |
Regulatory Assets [Line Items] |  |
Total Regulatory Assets | 71,900 | 64,100
PSNH | Regulatory Tracker Mechanisms |  |
Regulatory Assets [Line Items] |  |
Total Regulatory Assets | 76,800 | 95,300
PSNH | Derivative Liabilities |  |
Regulatory Assets [Line Items] |  |
Total Regulatory Assets | 0 | 0
PSNH | Goodwill-related |  |
Regulatory Assets [Line Items] |  |
Total Regulatory Assets | 0 | 0
PSNH | Asset Retirement Obligations |  |
Regulatory Assets [Line Items] |  |
Total Regulatory Assets | 4,000 | 3,900
PSNH | Other Regulatory Assets |  |
Regulatory Assets [Line Items] |  |
Total Regulatory Assets | $ 16,900 | $ 20,900

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