Marketable Securities (Schedule of Fair Value and Gross Unrealized Losses) (Details) - USD ($) $ in Thousands	Dec. 29, 2018	Sep. 30, 2018
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than Twelve Months, Fair Value	$ 24,259	$ 24,286
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss	(124)	(71)
Available-for-sale Securities, Continuous Unrealized Loss Position, Twelve Months or Longer, Fair Value	28,910	31,665
Available-for-sale Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss	(281)	(342)
Available-for-sale Securities, Continuous Unrealized Loss Position, Fair Value	53,169	55,951
Available-for-sale Securities, Continuous Unrealized Loss Position, Accumulated Loss	(405)	(413)
Certificates of deposit
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than Twelve Months, Fair Value	0	219
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss	0	(1)
Available-for-sale Securities, Continuous Unrealized Loss Position, Twelve Months or Longer, Fair Value	220	0
Available-for-sale Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss	(1)	0
Available-for-sale Securities, Continuous Unrealized Loss Position, Fair Value	220	219
Available-for-sale Securities, Continuous Unrealized Loss Position, Accumulated Loss	(1)	(1)
Commercial paper
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than Twelve Months, Fair Value	1,956
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss	(5)
Available-for-sale Securities, Continuous Unrealized Loss Position, Twelve Months or Longer, Fair Value	0
Available-for-sale Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss	0
Available-for-sale Securities, Continuous Unrealized Loss Position, Fair Value	1,956
Available-for-sale Securities, Continuous Unrealized Loss Position, Accumulated Loss	(5)
Corporate notes/bonds
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than Twelve Months, Fair Value	22,303	24,067
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss	(119)	(70)
Available-for-sale Securities, Continuous Unrealized Loss Position, Twelve Months or Longer, Fair Value	27,695	30,670
Available-for-sale Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss	(275)	(333)
Available-for-sale Securities, Continuous Unrealized Loss Position, Fair Value	49,998	54,737
Available-for-sale Securities, Continuous Unrealized Loss Position, Accumulated Loss	(394)	(403)
U.S. government agency securities
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than Twelve Months, Fair Value	0	0
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss	0	0
Available-for-sale Securities, Continuous Unrealized Loss Position, Twelve Months or Longer, Fair Value	995	995
Available-for-sale Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss	(5)	(9)
Available-for-sale Securities, Continuous Unrealized Loss Position, Fair Value	995	995
Available-for-sale Securities, Continuous Unrealized Loss Position, Accumulated Loss	$ (5)	$ (9)