Marketable Securities (Schedule of Fair Value and Gross Unrealized Losses) (Details) - USD ($) $ in Thousands	Jun. 29, 2019	Sep. 30, 2018
Debt Securities, Available-for-sale [Line Items]
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than Twelve Months, Fair Value	$ 3,519	$ 24,286
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss	(6)	(71)
Available-for-sale Securities, Continuous Unrealized Loss Position, Twelve Months or Longer, Fair Value	24,398	31,665
Available-for-sale Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss	(53)	(342)
Available-for-sale Securities, Continuous Unrealized Loss Position, Fair Value	27,917	55,951
Available-for-sale Securities, Continuous Unrealized Loss Position, Accumulated Loss	(59)	(413)
Certificates of deposit
Debt Securities, Available-for-sale [Line Items]
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than Twelve Months, Fair Value		219
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss		(1)
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		219
Available-for-sale Securities, Continuous Unrealized Loss Position, Accumulated Loss		(1)
Corporate notes/bonds
Debt Securities, Available-for-sale [Line Items]
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than Twelve Months, Fair Value	3,519	24,067
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss	(6)	(70)
Available-for-sale Securities, Continuous Unrealized Loss Position, Twelve Months or Longer, Fair Value	24,398	30,670
Available-for-sale Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss	(53)	(333)
Available-for-sale Securities, Continuous Unrealized Loss Position, Fair Value	27,917	54,737
Available-for-sale Securities, Continuous Unrealized Loss Position, Accumulated Loss	$ (59)	(403)
U.S. government agency securities
Debt Securities, Available-for-sale [Line Items]
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than Twelve Months, Fair Value		0
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss		0
Available-for-sale Securities, Continuous Unrealized Loss Position, Twelve Months or Longer, Fair Value		995
Available-for-sale Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss		(9)
Available-for-sale Securities, Continuous Unrealized Loss Position, Fair Value		995
Available-for-sale Securities, Continuous Unrealized Loss Position, Accumulated Loss		$ (9)