Annual report pursuant to Section 13 and 15(d)

Income Taxes (Details 2)

v2.4.1.9
Income Taxes (Details 2) (USD $)
12 Months Ended
Mar. 31, 2015
Mar. 31, 2014
Mar. 31, 2013
NOL and tax credit carry forwards for federal and state income tax purposes      
Period of time from grant date during which a tax benefit may be generated from a premature disposition of option shares exercised 2 years    
Period of time from exercise date during which a tax benefit may be generated from a premature disposition of option shares exercised 1 year    
Federal and state NOL carryforwards generated by disqualifying dispositions of stock options and exercises of nonqualified stock options $ 27,700,000cpst_FederalAndStateNetOperatingLossCarryforwardsGeneratedByDisqualifyingDispositionsOfStockOptionsAndExercisesOfNonqualifiedStockOptions    
Tax benefits associated with the stock options excluded from the provision for income taxes and credited directly to additional paid-in-capital 10,000,000us-gaap_IncomeTaxEffectsAllocatedDirectlyToEquityEmployeeStockOptions    
Other disclosures      
Interest or penalties related to unrecognized tax benefits 0us-gaap_UnrecognizedTaxBenefitsIncomeTaxPenaltiesAndInterestAccrued 0us-gaap_UnrecognizedTaxBenefitsIncomeTaxPenaltiesAndInterestAccrued  
Unrecognized tax benefits, that if recognized, would affect the effective tax rate 2,600,000us-gaap_UnrecognizedTaxBenefitsThatWouldImpactEffectiveTaxRate 2,400,000us-gaap_UnrecognizedTaxBenefitsThatWouldImpactEffectiveTaxRate  
Reconciliation of the beginning and ending amount of total gross unrecognized tax benefits      
Balance at the beginning of the period 2,441,000us-gaap_UnrecognizedTaxBenefits 2,418,000us-gaap_UnrecognizedTaxBenefits 2,148,000us-gaap_UnrecognizedTaxBenefits
Gross decrease related to prior year tax positions   (93,000)us-gaap_UnrecognizedTaxBenefitsDecreasesResultingFromPriorPeriodTaxPositions (100,000)us-gaap_UnrecognizedTaxBenefitsDecreasesResultingFromPriorPeriodTaxPositions
Gross increase related to prior year tax positions 11,000us-gaap_UnrecognizedTaxBenefitsIncreasesResultingFromPriorPeriodTaxPositions 1,000us-gaap_UnrecognizedTaxBenefitsIncreasesResultingFromPriorPeriodTaxPositions 222,000us-gaap_UnrecognizedTaxBenefitsIncreasesResultingFromPriorPeriodTaxPositions
Gross increase related to current year tax positions 112,000us-gaap_UnrecognizedTaxBenefitsIncreasesResultingFromCurrentPeriodTaxPositions 115,000us-gaap_UnrecognizedTaxBenefitsIncreasesResultingFromCurrentPeriodTaxPositions 148,000us-gaap_UnrecognizedTaxBenefitsIncreasesResultingFromCurrentPeriodTaxPositions
Balance at the end of the period 2,564,000us-gaap_UnrecognizedTaxBenefits 2,441,000us-gaap_UnrecognizedTaxBenefits 2,418,000us-gaap_UnrecognizedTaxBenefits
Maximum      
NOL and tax credit carry forwards for federal and state income tax purposes      
Annual limitation amount on utilization of the NOLs and tax credits 57,300,000cpst_AnnualLimitationAmountOnNetOperatingLossesAndTaxCreditsUtilization
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= us-gaap_MaximumMember
   
Federal      
NOL and tax credit carry forwards for federal and state income tax purposes      
Deferred tax assets related to research and development credits 9,700,000us-gaap_DeferredTaxAssetsTaxCreditCarryforwardsResearch
/ us-gaap_IncomeTaxAuthorityAxis
= us-gaap_InternalRevenueServiceIRSMember
9,200,000us-gaap_DeferredTaxAssetsTaxCreditCarryforwardsResearch
/ us-gaap_IncomeTaxAuthorityAxis
= us-gaap_InternalRevenueServiceIRSMember
 
NOL 625,918,000us-gaap_OperatingLossCarryforwards
/ us-gaap_IncomeTaxAuthorityAxis
= us-gaap_InternalRevenueServiceIRSMember
   
Tax credit carry forwards 9,684,000us-gaap_TaxCreditCarryforwardAmount
/ us-gaap_IncomeTaxAuthorityAxis
= us-gaap_InternalRevenueServiceIRSMember
   
State      
NOL and tax credit carry forwards for federal and state income tax purposes      
Deferred tax assets related to research and development credits 9,500,000us-gaap_DeferredTaxAssetsTaxCreditCarryforwardsResearch
/ us-gaap_IncomeTaxAuthorityAxis
= us-gaap_StateAndLocalJurisdictionMember
9,600,000us-gaap_DeferredTaxAssetsTaxCreditCarryforwardsResearch
/ us-gaap_IncomeTaxAuthorityAxis
= us-gaap_StateAndLocalJurisdictionMember
 
NOL 202,275,000us-gaap_OperatingLossCarryforwards
/ us-gaap_IncomeTaxAuthorityAxis
= us-gaap_StateAndLocalJurisdictionMember
   
Tax credit carry forwards $ 9,176,000us-gaap_TaxCreditCarryforwardAmount
/ us-gaap_IncomeTaxAuthorityAxis
= us-gaap_StateAndLocalJurisdictionMember