Short-Term Investments (Details) - USD ($) $ in Thousands	3 Months Ended
	Dec. 30, 2017	Dec. 31, 2016	Sep. 30, 2017
Available-for-sale Securities, Amortized Cost Basis [Abstract]
Cash and cash equivalents, Cost Basis	$ 385,710		$ 443,066
Cash and cash equivalent, Unrealized Gains	25		0
Cash and cash equivalent, Unrealized Losses	0		0
Cash and cash equivalents	385,735	$ 360,217	443,066
Proceeds from sale of available-for-sale securities	2,400	$ 100
Short-term investments:
Available-for-sale Securities, Amortized Cost Basis [Abstract]
Available-for-sale securities: Cost Basis	37,672		32,464
Available-for-sale Debt Securities, Accumulated Gross Unrealized Gain, before Tax	44		47
Available-for-sale Debt Securities, Accumulated Gross Unrealized Loss, before Tax	(5)		(1)
Available-for-sale securities: Fair Value	37,711		32,510
Short-term investments: | Commercial paper
Available-for-sale Securities, Amortized Cost Basis [Abstract]
Available-for-sale securities: Cost Basis	9,957
Available-for-sale Debt Securities, Accumulated Gross Unrealized Gain, before Tax	0
Available-for-sale Debt Securities, Accumulated Gross Unrealized Loss, before Tax	0
Available-for-sale securities: Fair Value	9,957
Short-term investments: | U.S. Treasury and agency obligations
Available-for-sale Securities, Amortized Cost Basis [Abstract]
Available-for-sale securities: Cost Basis	18,588		21,074
Available-for-sale Debt Securities, Accumulated Gross Unrealized Gain, before Tax	19		13
Available-for-sale Debt Securities, Accumulated Gross Unrealized Loss, before Tax	(2)		0
Available-for-sale securities: Fair Value	18,605		21,087
Short-term investments: | Corporate notes and obligations
Available-for-sale Securities, Amortized Cost Basis [Abstract]
Available-for-sale securities: Cost Basis	9,127		11,390
Available-for-sale Debt Securities, Accumulated Gross Unrealized Gain, before Tax	25		34
Available-for-sale Debt Securities, Accumulated Gross Unrealized Loss, before Tax	(3)		(1)
Available-for-sale securities: Fair Value	$ 9,149		$ 11,423