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Fair Value Measurements (Narrative) (Details) (USD $)
In Millions, unless otherwise specified
Mar. 31, 2015
Sep. 30, 2014
Foreign    
Segment Reporting Information [Line Items]    
Cash and cash equivalents, and short-term investments $ 75.9us-gaap_CashCashEquivalentsAndShortTermInvestments
/ us-gaap_StatementGeographicalAxis
= issi_ForeignMember
$ 71.9us-gaap_CashCashEquivalentsAndShortTermInvestments
/ us-gaap_StatementGeographicalAxis
= issi_ForeignMember
China    
Segment Reporting Information [Line Items]    
Cash and cash equivalents, and short-term investments $ 1.0us-gaap_CashCashEquivalentsAndShortTermInvestments
/ us-gaap_StatementGeographicalAxis
= country_CN