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401(k) Plan - Additional Information (Detail) (401 (k) Plan [Member], USD $)
 12 Months Ended
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 $ 184,000us-gaap_DefinedContributionPlanEmployerDiscretionaryContributionAmount
/ us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis
= stem_FourZeroOneKPlanMember		 $ 132,000us-gaap_DefinedContributionPlanEmployerDiscretionaryContributionAmount
/ us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis
= stem_FourZeroOneKPlanMember		 $ 130,000us-gaap_DefinedContributionPlanEmployerDiscretionaryContributionAmount
/ us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis
= stem_FourZeroOneKPlanMember