Balance Sheet Components - Unbilled Receivables and Deferred Revenue from Contracts with Customers (Details) - USD ($) $ in Millions	3 Months Ended	12 Months Ended
	Dec. 30, 2016	Sep. 28, 2018	Sep. 29, 2017	Sep. 30, 2016
Accounts, Notes, Loans and Financing Receivable [Line Items]
Unbilled receivables - long-term (1)		$ 36.3	$ 10.9
Unbilled receivables - current		362.8	259.1
Deferred revenues - current		(729.7)	(755.4)
Deferred revenues - long-term (2)		(38.6)	(7.2)
Total net unbilled receivables (deferred revenues)		(369.2)	(492.6)
Increase in unbilled receivables net of deferred revenues		123.4
Deferred revenues recognized		407.0	468.9
Provision for doubtful accounts receivable	$ 37.8	4.0	$ 43.7	$ 3.5
C P T C
Accounts, Notes, Loans and Financing Receivable [Line Items]
Provision for doubtful accounts receivable		$ 17.2