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Subsequent Events - Additional Information (Detail) (Subsequent Events [Member], USD $)
 1 Months Ended
Apr. 30, 2015
Subsequent Event [Line Items]  
Proceeds from issuance of common stock and warrants $ 25,000,000stem_ProceedsFromIssuanceOfCommonStockAndWarrants
Common stock issued in public offering 35,715,000us-gaap_StockIssuedDuringPeriodSharesNewIssues
Exercise price of warrants $ 0.85us-gaap_ClassOfWarrantOrRightExercisePriceOfWarrantsOrRights1
Warrant expiration period 5 years
Over-Allotments [Member]
  
Subsequent Event [Line Items]  
Common stock issued in public offering 5,357,250us-gaap_StockIssuedDuringPeriodSharesNewIssues
/ us-gaap_SubsequentEventTypeAxis
= us-gaap_SubsequentEventMember
/ us-gaap_SubsidiarySaleOfStockAxis
= us-gaap_OverAllotmentOptionMember
Number of securities callable by warrants 4,017,938us-gaap_ClassOfWarrantOrRightNumberOfSecuritiesCalledByWarrantsOrRights
/ us-gaap_SubsequentEventTypeAxis
= us-gaap_SubsequentEventMember
/ us-gaap_SubsidiarySaleOfStockAxis
= us-gaap_OverAllotmentOptionMember
Warrants issued for purchase of additional common shares 4,017,938stem_WarrantsIssuedToPurchaseCommonStockShares
/ us-gaap_SubsequentEventTypeAxis
= us-gaap_SubsequentEventMember
/ us-gaap_SubsidiarySaleOfStockAxis
= us-gaap_OverAllotmentOptionMember
Common Stock [Member]
  
Subsequent Event [Line Items]  
Number of securities callable by each warrant or right warrants 0.75us-gaap_ClassOfWarrantOrRightNumberOfSecuritiesCalledByEachWarrantOrRight
/ us-gaap_StatementEquityComponentsAxis
= us-gaap_CommonStockMember
/ us-gaap_SubsequentEventTypeAxis
= us-gaap_SubsequentEventMember