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Stock-Based Compensation - Summary of Changes in Restricted Stock Unit Activity (Parenthetical) (Detail)
 3 Months Ended
Mar. 31, 2015
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]  
Restricted stock units vest description 349,518 of these restricted stock units vest and convert into shares of our common stock after one year from the date of grant. 2,586,750 of these restricted stock units will vest and convert into shares of our common stock over a three year period from the date of grant; one-third of the award will vest on each grant date anniversary following the grant. 4,354,500 of these restricted stock units are performance based and vest on achievement of predefined milestones.
Restricted Stock [Member]  
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]  
Number of restricted units would vest over vesting period 349,518stem_NumberOfRestrictedUnitWouldVestOverVestingPeriod
/ us-gaap_AwardTypeAxis
= us-gaap_RestrictedStockMember
Restricted stock units vest and convert into shares of our common stock 1 year
Restricted Stock Two [Member]  
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]  
Number of restricted units would vest over vesting period 2,586,750stem_NumberOfRestrictedUnitWouldVestOverVestingPeriod
/ us-gaap_AwardTypeAxis
= stem_RestrictedStockTwoMember
Restricted stock units vest and convert into shares of our common stock 3 years
Restricted stock units vested description after specified period 0.3333stem_RestrictedStockUnitsVestedDescriptionAfterSpecifiedPeriod
/ us-gaap_AwardTypeAxis
= stem_RestrictedStockTwoMember
Performance Based Restricted Stock Units [Member]  
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]  
Number of restricted units vest and convert into common stock over next fiscal year 4,354,500stem_NumberOfRestrictedUnitVestOverNextFiscalYear
/ us-gaap_AwardTypeAxis
= us-gaap_PerformanceSharesMember