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SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES - Non-cash Investing Activities (Details) - USD ($) $ in Millions | 3 Months Ended
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Mar. 31, 2025 | Mar. 31, 2024
Schedule of Supplemental Cash Flow [Line Items] |  |
Plant additions included in accounts payable | $ 427.2 | $ 453.6
CL&P |  |
Schedule of Supplemental Cash Flow [Line Items] |  |
Plant additions included in accounts payable | 82.3 | 112.8
NSTAR Electric |  |
Schedule of Supplemental Cash Flow [Line Items] |  |
Plant additions included in accounts payable | 172.8 | 132.7
PSNH |  |
Schedule of Supplemental Cash Flow [Line Items] |  |
Plant additions included in accounts payable | $ 58.2 | $ 75.2

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- Definition Future cash outflow to pay for purchases of fixed assets that have occurred. + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Name Accounting Standards Codification -Topic 230 -SubTopic 10 -Section 50 -Paragraph 4 -Publisher FASB -URI https://asc.fasb.org/1943274/2147482913/230-10-50-4 Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef -Name Accounting Standards Codification -Topic 230 -SubTopic 10 -Section 50 -Paragraph 3 -Publisher FASB -URI https://asc.fasb.org/1943274/2147482913/230-10-50-3 Reference 3: http://fasb.org/us-gaap/role/ref/legacyRef -Name Accounting Standards Codification -Topic 230 -SubTopic 10 -Section 50 -Paragraph 5 -Publisher FASB -URI https://asc.fasb.org/1943274/2147482913/230-10-50-5 + Details
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