SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES - Non-cash Investing Activities (Details) - USD ($) $ in Millions	6 Months Ended
Jun. 30, 2021	Jun. 30, 2020
Schedule of Supplemental Cash Flow [Line Items]
Plant additions included in accounts payable	$ 344.8	$ 336.1
CL&P
Schedule of Supplemental Cash Flow [Line Items]
Plant additions included in accounts payable	66.4	95.0
NSTAR Electric
Schedule of Supplemental Cash Flow [Line Items]
Plant additions included in accounts payable	103.8	75.5
PSNH
Schedule of Supplemental Cash Flow [Line Items]
Plant additions included in accounts payable	$ 33.9	$ 48.3

Future cash outflow to pay for purchases of fixed assets that have occurred.

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

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