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SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES - Non-cash Investing Activities (Details) - USD ($) $ in Millions | 9 Months Ended
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Sep. 30, 2023 | Sep. 30, 2022
Schedule of Supplemental Cash Flow [Line Items] |  |
Plant additions included in accounts payable | $ 462.4 | $ 394.1
CL&P |  |
Schedule of Supplemental Cash Flow [Line Items] |  |
Plant additions included in accounts payable | 86.3 | 96.7
NSTAR Electric |  |
Schedule of Supplemental Cash Flow [Line Items] |  |
Plant additions included in accounts payable | 135.9 | 94.5
PSNH |  |
Schedule of Supplemental Cash Flow [Line Items] |  |
Plant additions included in accounts payable | $ 71.9 | $ 49.7

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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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