Organization and Significant Accounting Policies (Schedule of Comprehensive Income (Loss)) (Details) (USD $) In Thousands, unless otherwise specified	3 Months Ended										12 Months Ended
	Sep. 30, 2012		Jun. 30, 2012	Mar. 31, 2012	Dec. 31, 2011	Sep. 30, 2011		Jun. 30, 2011	Mar. 31, 2011	Dec. 31, 2010	Sep. 30, 2012	Sep. 30, 2011	Sep. 30, 2010
Organization, Consolidation and Presentation of Financial Statements [Abstract]
Consolidated net income (loss)	$ (13,118)	[1]	$ 3,147	$ 3,597	$ 3,769	$ 35,056	[2]	$ 8,095	$ 5,758	$ 7,214	$ (2,605)	$ 56,123	$ 42,755
Change in cumulative translation adjustment:
Changes arising during current year											3,932	1,850	1,839
Reclassification for gain included in net income (loss)											0	0	0
Change in unrealized gain (loss) on investments:
Changes arising during current year											(1,523)	57	746
Reclassification for gain (loss) included in net income (loss)											1,213	0	(1,673)
Change in retirement plan transition asset											(196)	(50)	(47)
Change in retirement plan actuarial losses											225	(823)	(1,807)
Total comprehensive income (loss)											1,046	57,157	41,813
Comprehensive income (loss) attributable to noncontrolling interest											(113)	(166)	(559)
Comprehensive income (loss) attributable to ISSI											$ 933	$ 56,991	$ 41,254

[1] In the September 2012 quarter, the Company recorded a charge of approximately $14.3 million for the impairment of certain tangible and intangible assets related to the acquisition of Si En and a charge of approximately $2.3 million to write-down its investment in SMIC due to the decline in fair market value being considered other than temporary.

[2] In the September 2011 quarter, the Company recorded an income tax benefit of approximately $28.1 million from the release of the valuation allowance for certain deferred tax assets.