SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES - Non-cash Investing Activities (Details) - USD ($) $ in Millions	3 Months Ended
Mar. 31, 2025	Mar. 31, 2024
Schedule of Supplemental Cash Flow [Line Items]
Plant additions included in accounts payable	$ 427.2	$ 453.6
CL&P
Schedule of Supplemental Cash Flow [Line Items]
Plant additions included in accounts payable	82.3	112.8
NSTAR Electric
Schedule of Supplemental Cash Flow [Line Items]
Plant additions included in accounts payable	172.8	132.7
PSNH
Schedule of Supplemental Cash Flow [Line Items]
Plant additions included in accounts payable	$ 58.2	$ 75.2

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

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

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