Condensed Consolidated Statements of Income (USD $) In Thousands, except Per Share data, unless otherwise specified	3 Months Ended		6 Months Ended
	Mar. 31, 2015	Mar. 31, 2014	Mar. 31, 2015	Mar. 31, 2014
Income Statement [Abstract]
Net sales	$ 80,104	$ 80,868	$ 161,014	$ 159,991
Cost of sales	52,056	53,118	103,821	106,712
Gross profit	28,048	27,750	57,193	53,279
Operating expenses:
Research and development	12,423	10,827	24,671	21,385
Selling, general and administrative	14,177	11,469	27,074	22,903
Total operating expenses	26,600	22,296	51,745	44,288
Operating income	1,448	5,454	5,448	8,991
Interest and other income (expense), net	365	375	527	806
Gain on sale of investments	0	6,039	0	9,160
Income before income taxes	1,813	11,868	5,975	18,957
Provision for income taxes	682	2,910	1,608	4,553
Consolidated net income	1,131	8,958	4,367	14,404
Net income attributable to noncontrolling interests	(15)	(146)	(65)	(157)
Net income attributable to ISSI	$ 1,116	$ 8,812	$ 4,302	$ 14,247
Basic net income per share (in dollars per share)	$ 0.04	$ 0.30	$ 0.14	$ 0.48
Shares used in basic per share calculation	31,544	29,818	31,267	29,568
Diluted net income per share (in dollars per share)	$ 0.03	$ 0.28	$ 0.13	$ 0.46
Shares used in diluted per share calculation	33,270	31,244	32,936	30,986