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SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES - Energy Storage Systems (Details)	12 Months Ended
	Dec. 31, 2020
Energy Storage Systems
Property, Plant and Equipment [Line Items]
Estimated useful lives	10 years

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- Definition

Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

+ References

No definition available.

+ Details

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Namespace Prefix:	us-gaap_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

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- Definition

Useful life of long lived, physical assets used in the normal conduct of business and not intended for resale, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents the reported fact of one year, five months, and thirteen days. Examples include, but not limited to, land, buildings, machinery and equipment, office equipment, furniture and fixtures, and computer equipment.

+ References

No definition available.

+ Details

Name:	us-gaap_PropertyPlantAndEquipmentUsefulLife
Namespace Prefix:	us-gaap_
Data Type:	xbrli:durationItemType
Balance Type:	na
Period Type:	duration

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- Details

Name:	us-gaap_PropertyPlantAndEquipmentByTypeAxis=stem_EnergyStorageSystemsMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type: