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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, 2021 | Sep. 30, 2020
Schedule of Supplemental Cash Flow [Line Items] |  |
Plant additions included in accounts payable | $ 359.6 | $ 357.4
CL&P |  |
Schedule of Supplemental Cash Flow [Line Items] |  |
Plant additions included in accounts payable | 75.3 | 97.8
NSTAR Electric |  |
Schedule of Supplemental Cash Flow [Line Items] |  |
Plant additions included in accounts payable | 94.2 | 92.6
PSNH |  |
Schedule of Supplemental Cash Flow [Line Items] |  |
Plant additions included in accounts payable | $ 32.8 | $ 50.0

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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 http://asc.fasb.org/extlink&oid=123431023&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 http://asc.fasb.org/extlink&oid=123431023&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 http://asc.fasb.org/extlink&oid=123431023&loc=d3e4304-108586 + Details
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