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401(k) Plan - Additional Information (Detail) (401 (k) Plan [Member], USD $) | 12 Months Ended
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Dec. 31, 2014 | Dec. 31, 2013 | Dec. 31, 2012
401 (k) Plan [Member] |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Employer Contribution to plan | 50.00%us-gaap_DefinedContributionPlanEmployerMatchingContributionPercent / us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis = stem_FourZeroOneKPlanMember |  |
Percentage of employee eligible for compensation in the form of shares | 6.00%stem_DefinedContributionPlanEligibleCompensationEmployerMaximumContributionPercentage / us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis = stem_FourZeroOneKPlanMember |  |
Contributions expense | $ 184,000 us-gaap_DefinedContributionPlanEmployerDiscretionaryContributionAmount / us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis = stem_FourZeroOneKPlanMember | $ 132,000 us-gaap_DefinedContributionPlanEmployerDiscretionaryContributionAmount / us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis = stem_FourZeroOneKPlanMember | $ 130,000 us-gaap_DefinedContributionPlanEmployerDiscretionaryContributionAmount / us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis = stem_FourZeroOneKPlanMember

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- Definition Defined contribution plan eligible compensation employer maximum contribution percentage. + References No definition available. + Details
Name: | stem_DefinedContributionPlanEligibleCompensationEmployerMaximumContributionPercentage
Namespace Prefix: | stem_
Data Type: | num:percentItemType
Balance Type: | na
Period Type: | duration

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- Details
Name: | us-gaap_DefinedBenefitPlanDisclosureLineItems
Namespace Prefix: | us-gaap_
Data Type: | xbrli:stringItemType
Balance Type: | na
Period Type: | duration

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- Definition Amount of discretionary contributions made by an employer to a defined contribution plan. + References No definition available. + Details
Name: | us-gaap_DefinedContributionPlanEmployerDiscretionaryContributionAmount
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | duration

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- Definition Percentage of employees' gross pay for which the employer contributes a matching contribution to a defined contribution plan. + References No definition available. + Details
Name: | us-gaap_DefinedContributionPlanEmployerMatchingContributionPercent
Namespace Prefix: | us-gaap_
Data Type: | num:percentItemType
Balance Type: | na
Period Type: | duration