Revenue from Contracts with Customers (Impact of ASC 606 on Balance Sheet) (Details) - USD ($) $ in Thousands	Jun. 29, 2019	Mar. 30, 2019	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Sep. 30, 2017
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Cash and cash equivalents	$ 267,862		$ 259,946
Short-term marketable securities	33,073		25,836
Accounts receivable, net of allowance for doubtful accounts of $822 and $607 at June 29, 2019 and September 30, 2018, respectively	321,426		129,297
Prepaid expenses	69,819		48,997
Other current assets	60,483		169,708
Total current assets	752,663		633,784
Property and equipment, net	107,752		80,613
Goodwill	1,245,084		1,182,457
Acquired intangible assets, net	183,180		200,202
Long-term marketable securities	21,553		30,115
Deferred tax assets	189,371		165,566
Other assets	148,998		36,285
Total assets	2,648,601		2,329,022
Accounts payable	44,065		53,473
Accrued expenses and other current liabilities	95,790		74,388
Accrued compensation and benefits	78,854		101,784
Accrued income taxes	8,222		18,044
Deferred revenue	374,291		487,590
Total current liabilities	601,222		735,279
Long-term debt	698,916		643,268
Deferred tax liabilities	37,354		5,589
Deferred revenue	8,288		11,852
Other liabilities	87,226		58,445
Total liabilities	1,433,006		1,454,433
Commitments and contingencies (Note 14)
Preferred stock	0		0
Common stock	1,151		1,180
Additional paid-in capital	1,504,512		1,558,403
Accumulated deficit	(197,056)		(599,409)
Accumulated other comprehensive loss	(93,012)		(85,585)
Total stockholders’ equity	1,215,595	$ 1,247,708	874,589	$ 834,385	$ 927,724	$ 885,436
Total liabilities and stockholders’ equity	2,648,601		$ 2,329,022
Calculated under Revenue Guidance in Effect before Topic 606 [Member]
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Cash and cash equivalents	267,862
Short-term marketable securities	33,073
Accounts receivable, net of allowance for doubtful accounts of $822 and $607 at June 29, 2019 and September 30, 2018, respectively	111,165
Prepaid expenses	75,504
Other current assets	139,997
Total current assets	627,601
Property and equipment, net	107,752
Goodwill	1,245,084
Acquired intangible assets, net	183,180
Long-term marketable securities	21,553
Deferred tax assets	222,477
Other assets	41,121
Total assets	2,448,768
Accounts payable	44,065
Accrued expenses and other current liabilities	71,506
Accrued compensation and benefits	78,854
Accrued income taxes	4,135
Deferred revenue	542,888
Total current liabilities	741,448
Long-term debt	698,916
Deferred tax liabilities	6,635
Deferred revenue	8,045
Other liabilities	87,226
Total liabilities	1,542,270
Preferred stock	0
Common stock	1,151
Additional paid-in capital	1,504,512
Accumulated deficit	(508,225)
Accumulated other comprehensive loss	(90,940)
Total stockholders’ equity	906,498
Total liabilities and stockholders’ equity	$ 2,448,768