Per Share Data (Calculation of Basic and Diluted EPS) (Details) (USD $) In Thousands, except Per Share data, unless otherwise specified	3 Months Ended
Dec. 31, 2014	Dec. 31, 2013
Earnings Per Share [Abstract]
Net income attributable to ISSI	$ 3,186us-gaap_NetIncomeLoss	$ 5,435us-gaap_NetIncomeLoss
Denominator for basic net income per share:
Weighted average common shares outstanding	30,990	29,318
Dilutive stock options and awards	1,531	1,399
Denominator for diluted net income per share	32,521	30,717
Basic net income per share (in dollars per share)	$ 0.10us-gaap_EarningsPerShareBasic	$ 0.19us-gaap_EarningsPerShareBasic
Diluted net income per share (in dollars per share)	$ 0.10us-gaap_EarningsPerShareDiluted	$ 0.18us-gaap_EarningsPerShareDiluted

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The amount of net income (loss) for the period per each share of common stock or unit outstanding during the reporting period.

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The amount of net income (loss) for the period available to each share of common stock or common unit outstanding during the reporting period and to each share or unit that would have been outstanding assuming the issuance of common shares or units for all dilutive potential common shares or units outstanding during the reporting period.

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The portion of profit or loss for the period, net of income taxes, which is attributable to the parent.

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The sum of dilutive potential common shares or units used in the calculation of the diluted per-share or per-unit computation.

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The average number of shares or units issued and outstanding that are used in calculating diluted EPS or earnings per unit (EPU), determined based on the timing of issuance of shares or units in the period.

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Number of [basic] shares or units, after adjustment for contingently issuable shares or units and other shares or units not deemed outstanding, determined by relating the portion of time within a reporting period that common shares or units have been outstanding to the total time in that period.

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