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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, 2023 | Mar. 31, 2022
Schedule of Supplemental Cash Flow [Line Items] |  |
Plant additions included in accounts payable | $ 400.8 | $ 385.6
CL&P |  |
Schedule of Supplemental Cash Flow [Line Items] |  |
Plant additions included in accounts payable | 101.9 | 102.1
NSTAR Electric |  |
Schedule of Supplemental Cash Flow [Line Items] |  |
Plant additions included in accounts payable | 117.4 | 85.8
PSNH |  |
Schedule of Supplemental Cash Flow [Line Items] |  |
Plant additions included in accounts payable | $ 54.6 | $ 49.5

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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 -Publisher FASB -Name Accounting Standards Codification -Topic 230 -SubTopic 10 -Section 50 -Paragraph 5 -URI https://asc.fasb.org/extlink&oid=126999549&loc=d3e4332-108586 Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 230 -SubTopic 10 -Section 50 -Paragraph 4 -URI https://asc.fasb.org/extlink&oid=126999549&loc=d3e4313-108586 Reference 3: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 230 -SubTopic 10 -Section 50 -Paragraph 3 -URI https://asc.fasb.org/extlink&oid=126999549&loc=d3e4304-108586 + Details
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