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Stock-based Compensation (Narrative) (Details) (USD $)
In Thousands, except Share data, unless otherwise specified
3 Months Ended
Dec. 31, 2014
Dec. 31, 2013
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]    
Shares available for future grant under the 2007 Plan (shares) 1,532,000us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardNumberOfSharesAvailableForGrant  
Vesting period 4 years  
1995 Employee Stock Purchase Plan    
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]    
Percentage discount from fair market value for shares purchased under the Employee Stock Purchase Plan 85.00%us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardDiscountFromMarketPricePurchaseDate
/ us-gaap_PlanNameAxis
= issi_NineteenNinetyFiveEmployeeStockPurchasePlanMember
 
Offering period (years) 6 months  
Shares available for future issuance under the Employee Stock Purchase Plan (shares) 332,000us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardNumberOfSharesAuthorized
/ us-gaap_PlanNameAxis
= issi_NineteenNinetyFiveEmployeeStockPurchasePlanMember
 
Prior to October 1 2005    
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]    
Option expiration period (years) 10 years  
After October 1 2005    
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]    
Option expiration period (years) 7 years  
Minimum    
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]    
Option Cliff vesting period 6 months  
Maximum    
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]    
Option Cliff vesting period 1 year  
Restricted Stock Units (RSUs)    
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]    
Weighted-average fair value of grants $ 13.54us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsGrantsInPeriodWeightedAverageGrantDateFairValue
/ us-gaap_AwardTypeAxis
= us-gaap_RestrictedStockUnitsRSUMember
$ 10.96us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsGrantsInPeriodWeightedAverageGrantDateFairValue
/ us-gaap_AwardTypeAxis
= us-gaap_RestrictedStockUnitsRSUMember
Restricted Stock Units (RSUs) | Minimum    
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]    
Vesting period 2 years  
Restricted Stock Units (RSUs) | Maximum    
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]    
Vesting period 4 years  
SARS    
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]    
Vesting period 4 years  
Option expiration period (years) 7 years  
Total unrecognized stock-based compensation expense $ 7,100us-gaap_EmployeeServiceShareBasedCompensationNonvestedAwardsTotalCompensationCostNotYetRecognized
/ us-gaap_AwardTypeAxis
= us-gaap_StockAppreciationRightsSARSMember
 
Nonvested awards, total compensation cost not yet recognized, period for recognition (years) 3 years 4 months 13 days  
SARS | Minimum    
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]    
Option Cliff vesting period 6 months  
SARS | Maximum    
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]    
Option Cliff vesting period 1 year  
Options    
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]    
Total unrecognized stock-based compensation expense 10,000us-gaap_EmployeeServiceShareBasedCompensationNonvestedAwardsTotalCompensationCostNotYetRecognized
/ us-gaap_AwardTypeAxis
= us-gaap_EmployeeStockOptionMember
 
Nonvested awards, total compensation cost not yet recognized, period for recognition (years) 2 years 6 months  
ESPP    
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]    
Total unrecognized stock-based compensation expense $ 56us-gaap_EmployeeServiceShareBasedCompensationNonvestedAwardsTotalCompensationCostNotYetRecognized
/ us-gaap_AwardTypeAxis
= us-gaap_EmployeeStockMember
 
Nonvested awards, total compensation cost not yet recognized, period for recognition (years) 1 month