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SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES - Non-cash Investing Activities (Details) - USD ($) $ in Millions | 6 Months Ended
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Jun. 30, 2022 | Jun. 30, 2021
Schedule of Supplemental Cash Flow [Line Items] |  |
Plant additions included in accounts payable | $ 357.2 | $ 344.8
CL&P |  |
Schedule of Supplemental Cash Flow [Line Items] |  |
Plant additions included in accounts payable | 96.7 | 66.4
NSTAR Electric |  |
Schedule of Supplemental Cash Flow [Line Items] |  |
Plant additions included in accounts payable | 75.1 | 103.8
PSNH |  |
Schedule of Supplemental Cash Flow [Line Items] |  |
Plant additions included in accounts payable | $ 49.6 | $ 33.9

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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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