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SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES - Materials, Supplies, Natural Gas, and REC Inventory (Details) - USD ($) $ in Millions | Dec. 31, 2023 | Dec. 31, 2022
--- | --- | ---
Public Utilities, Inventory [Line Items] |  |
Total | $ 507.3 | $ 374.4
Materials and Supplies |  |
Public Utilities, Inventory [Line Items] |  |
Other inventory, capitalized costs, gross | 397.9 | 221.0
Natural Gas |  |
Public Utilities, Inventory [Line Items] |  |
Other inventory, capitalized costs, gross | 65.5 | 95.9
RECs |  |
Public Utilities, Inventory [Line Items] |  |
Other inventory, capitalized costs, gross | 43.9 | 57.5
CL&P |  |
Public Utilities, Inventory [Line Items] |  |
Total | 156.5 | 88.2
CL&P | Materials and Supplies |  |
Public Utilities, Inventory [Line Items] |  |
Other inventory, capitalized costs, gross | 156.2 | 88.2
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 | 0.3 | 0.0
NSTAR Electric |  |
Public Utilities, Inventory [Line Items] |  |
Total | 173.8 | 138.4
NSTAR Electric | Materials and Supplies |  |
Public Utilities, Inventory [Line Items] |  |
Other inventory, capitalized costs, gross | 130.8 | 81.0
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 | 43.0 | 57.4
PSNH |  |
Public Utilities, Inventory [Line Items] |  |
Total | 77.1 | 34.5
PSNH | Materials and Supplies |  |
Public Utilities, Inventory [Line Items] |  |
Other inventory, capitalized costs, gross | 76.5 | 34.4
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 | $ 0.6 | $ 0.1

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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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