COMMITMENTS AND CONTINGENCIES - Environmental Sites and Related Reserves (Details) $ in Millions	9 Months Ended	12 Months Ended
Sep. 30, 2021 USD ($) site	Dec. 31, 2020 USD ($) site
Site Contingency [Line Items]
Number of Sites \| site	62	63
Reserve \| $	$ 111.9	$ 102.4
CL&P
Site Contingency [Line Items]
Number of Sites \| site	14	15
Reserve \| $	$ 12.5	$ 12.3
NSTAR Electric
Site Contingency [Line Items]
Number of Sites \| site	12	12
Reserve \| $	$ 3.7	$ 4.7
PSNH
Site Contingency [Line Items]
Number of Sites \| site	9	9
Reserve \| $	$ 6.4	$ 7.1

Total costs accrued as of the balance sheet date for environmental loss contingencies.

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 450
-SubTopic 20
-Section 50
-Paragraph 1
-URI http://asc.fasb.org/extlink&oid=121557415&loc=d3e14326-108349

Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 450
-SubTopic 20
-Section 25
-Paragraph 2
-URI http://asc.fasb.org/extlink&oid=6395460&loc=d3e13647-108346

Reference 3: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 410
-SubTopic 30
-Section 50
-Paragraph 7
-URI http://asc.fasb.org/extlink&oid=6393242&loc=d3e13207-110859

Name:	us-gaap_AccrualForEnvironmentalLossContingencies
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant