Business Acquisitions (Schedule of Purchase Price Allocation) (Details) - USD ($) $ in Thousands	6 Months Ended
	Mar. 31, 2016	Sep. 30, 2015	Jun. 30, 2015	Mar. 31, 2015
Business Acquisition [Line Items]
Goodwill	$ 1,012,652			$ 571,271
Micrel Incorporated [Member]
Business Acquisition [Line Items]
Cash and cash equivalents	99,196	$ 99,196
Accounts receivable, net	14,096	12,296
Inventories	73,468	78,967
Prepaid expenses and other current assets	10,652	10,548
Property, plant and equipment, net	38,566	38,566
Goodwill	440,992	437,060
Purchased intangible assets	273,500	274,800
Other assets	4,268	4,268
Total assets acquired	954,738	955,701
Accounts payable	(11,068)	(11,068)
Other current liabilities	(31,641)	(30,241)
Deferred tax liability	(88,035)	(88,796)
Long-term income tax payable	(7,637)	(9,239)
Other long-term liabilities	(127)	(127)
Total liabilities assumed	(138,508)	(139,471)
Net assets acquired	816,230	$ 816,230
Business Combination, Provisional Information, Initial Accounting Incomplete, Adjustments [Abstract]
Accounts receivable, net	1,800
Inventories	(5,499)
Prepaid expenses and other current assets	104
Goodwill	3,932
Purchased intangible assets	(1,300)
Total assets acquired	(963)
Other current liabilities	(1,400)
Deferred tax liabilities	761
Long-term income tax payable	1,602
Total liabilities assumed	$ 963
ISSC Technologies Corporation [Member]
Business Acquisition [Line Items]
Cash and cash equivalents			$ 15,120
Short-term investments			27,063
Accounts receivable, net			8,792
Inventories			16,542
Prepaid expenses and other current assets			2,501
Property, plant and equipment, net			2,637
Goodwill			154,788
Purchased intangible assets			147,800
Other assets			1,370
Total assets acquired			376,613
Accounts payable			(9,860)
Other current liabilities			(16,535)
Deferred tax liability			(25,126)
Long-term income tax payable			(4,791)
Other long-term liabilities			(245)
Total liabilities assumed			(56,557)
Net assets acquired			320,056
Less: noncontrolling interest			(52,467)
Net assets acquired			$ 267,589
Supertex Inc. [Member]
Business Acquisition [Line Items]
Cash and cash equivalents				14,790
Short-term investments				140,984
Accounts receivable, net				7,047
Inventories				27,630
Prepaid expenses				1,493
Deferred tax assets				2,456
Other current assets				12,625
Property, plant and equipment, net				15,679
Goodwill				143,160
Purchased intangible assets				89,600
Other assets				325
Total assets acquired				455,789
Accounts payable				(8,481)
Other current liabilities				(19,224)
Deferred tax liability				(32,511)
Long-term income tax payable				(3,796)
Total liabilities assumed				(64,012)
Net assets acquired				$ 391,777