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SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES - Materials, Supplies, Natural Gas, and REC Inventory (Details) - USD ($) $ in Millions | Dec. 31, 2024 | Dec. 31, 2023
--- | --- | ---
Public Utilities, Inventory [Line Items] |  |
Total | $ 594.6 | $ 507.3
Materials and Supplies |  |
Public Utilities, Inventory [Line Items] |  |
Other inventory, capitalized costs, gross | 498.6 | 397.9
Natural Gas |  |
Public Utilities, Inventory [Line Items] |  |
Other inventory, capitalized costs, gross | 49.5 | 65.5
RECs |  |
Public Utilities, Inventory [Line Items] |  |
Other inventory, capitalized costs, gross | 46.5 | 43.9
CL&P |  |
Public Utilities, Inventory [Line Items] |  |
Total | 217.3 | 156.5
CL&P | Materials and Supplies |  |
Public Utilities, Inventory [Line Items] |  |
Other inventory, capitalized costs, gross | 217.3 | 156.2
CL&P | Natural Gas |  |
Public Utilities, Inventory [Line Items] |  |
Other inventory, capitalized costs, gross | 0.0 | 0.0
CL&P | RECs |  |
Public Utilities, Inventory [Line Items] |  |
Other inventory, capitalized costs, gross | 0.0 | 0.3
NSTAR Electric |  |
Public Utilities, Inventory [Line Items] |  |
Total | 220.6 | 173.8
NSTAR Electric | Materials and Supplies |  |
Public Utilities, Inventory [Line Items] |  |
Other inventory, capitalized costs, gross | 177.8 | 130.8
NSTAR Electric | Natural Gas |  |
Public Utilities, Inventory [Line Items] |  |
Other inventory, capitalized costs, gross | 0.0 | 0.0
NSTAR Electric | RECs |  |
Public Utilities, Inventory [Line Items] |  |
Other inventory, capitalized costs, gross | 42.8 | 43.0
PSNH |  |
Public Utilities, Inventory [Line Items] |  |
Total | 75.8 | 77.1
PSNH | Materials and Supplies |  |
Public Utilities, Inventory [Line Items] |  |
Other inventory, capitalized costs, gross | 72.1 | 76.5
PSNH | Natural Gas |  |
Public Utilities, Inventory [Line Items] |  |
Other inventory, capitalized costs, gross | 0.0 | 0.0
PSNH | RECs |  |
Public Utilities, Inventory [Line Items] |  |
Other inventory, capitalized costs, gross | $ 3.7 | $ 0.6

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- Definition Gross amount, as of the balance sheet date, of merchandise, goods, commodities, or supplies held for future sale or to be used in manufacturing, servicing or production process. + References Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef -Topic 210 -SubTopic 10 -Name Accounting Standards Codification -Section S99 -Paragraph 1 -Subparagraph (SX 210.5-02(6)) -Publisher FASB -URI https://asc.fasb.org/1943274/2147480566/210-10-S99-1 + Details
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- Definition Amount, before valuation and last-in first out (LIFO) reserves, of costs capitalized in inventory classified as other, expected to be sold, or consumed within one year or operating cycle, if longer. + References Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef -Topic 210 -SubTopic 10 -Name Accounting Standards Codification -Section S99 -Paragraph 1 -Subparagraph (SX 210.5-02(6)(a)) -Publisher FASB -URI https://asc.fasb.org/1943274/2147480566/210-10-S99-1 + Details
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