Quarterly Financial Information (Details) (USD $)	3 Months Ended														12 Months Ended
	Sep. 30, 2014	Jun. 30, 2014		Mar. 31, 2014		Dec. 31, 2013		Sep. 30, 2013		Jun. 30, 2013		Mar. 31, 2013		Dec. 31, 2012	Sep. 30, 2014	Sep. 30, 2013	Sep. 30, 2012
Schedule of Quarterly Financial Information [Line Items]
Net sales	$ 84,152,000	$ 84,834,000		$ 80,868,000		$ 79,123,000		$ 78,392,000		$ 77,788,000		$ 74,991,000		$ 76,399,000	$ 328,977,000	$ 307,570,000	$ 265,950,000
Gross profit	29,681,000	29,296,000		27,750,000		25,529,000		25,934,000		26,047,000		24,989,000		24,491,000	112,256,000	101,461,000	82,984,000
Operating income	5,351,000	6,376,000		5,454,000		3,537,000		4,693,000		4,635,000		3,951,000		3,379,000	20,718,000	16,658,000	5,631,000
Consolidated net income	2,858,000	6,227,000	[1]	8,958,000	[2]	5,446,000	[3]	4,730,000	[4]	7,246,000	[5]	3,280,000	[6]	2,488,000	23,489,000	17,744,000	(2,605,000)
Net income attributable to ISSI	2,832,000	6,186,000	[1]	8,812,000	[2]	5,435,000	[3]	4,698,000	[4]	7,064,000	[5]	3,287,000	[6]	2,499,000	23,265,000	17,548,000	(2,718,000)
Basic net income per share	$ 0.09	$ 0.20		$ 0.30		$ 0.19		$ 0.16		$ 0.25		$ 0.12		$ 0.09	$ 0.77	$ 0.62	$ (0.10)
Diluted net income (loss) per share	$ 0.09	$ 0.19		$ 0.28		$ 0.18		$ 0.15		$ 0.24		$ 0.11		$ 0.09	$ 0.74	$ 0.59	$ (0.10)
Dividends per common share paid	$ 0.06	$ 0		$ 0		$ 0
Market price range common stock:
Realized gain on sale of investment		2,100,000		6,000,000						7,300,000
Maximum
Market price range common stock:
Market price per share	$ 15.8	$ 15.95		$ 15.6		$ 12.43		$ 11.99		$ 11.31		$ 9.92		$ 9.73	$ 15.8	$ 11.99
Minimum
Market price range common stock:
Market price per share	$ 13.62	$ 12.78		$ 10.8		$ 10.52		$ 9.99		$ 8.31		$ 8.33		$ 8.18	$ 13.62	$ 9.99
Nanya
Market price range common stock:
Realized gain on sale of investment				3,900,000		3,100,000		2,900,000		7,000,000		2,100,000
(SMIC) common stock
Market price range common stock:
Realized gain on sale of investment				$ 2,100,000

[1] In the June 2014 quarter, the Company realized a gain of $2.1 million from the sale of its remaining freely-tradeable shares of Nanya common stock.

[2] In the March 2014 quarter, the Company realized a gain from the sale of investments of $6.0 million including $3.9 million from the sale of a portion of its shares of Nanya common stock and $2.1 million from the sale of its remaining shares of SMIC common stock.

[3] In the December 2013 quarter, the Company realized a gain of $3.1 million from the sale of a portion of its shares of Nanya common stock.

[4] In the September 2013 quarter, the Company realized a gain of $2.9 million from the sale of a portion of its shares of Nanya common stock.

[5] In the June 2013 quarter, the Company realized a gain from the sale of investments of $7.3 million including $7.0 million from the sale of a portion of its shares of Nanya common stock.

[6] In the March 2013 quarter, the Company realized a gain of $2.1 million from the sale of a portion of its shares of Nanya common stock.