Marketable Securities (Details) - USD ($) $ in Thousands	Dec. 30, 2017	Sep. 30, 2017
Schedule of Available-for-sale Securities [Line Items]
Amortized cost	$ 50,890	$ 50,458
Gross unrealized gains	0	2
Gross unrealized losses	(323)	(145)
Fair value	50,567	50,315
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than Twelve Months, Fair Value	22,893	15,494
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss	(184)	(43)
Available-for-sale Securities, Continuous Unrealized Loss Position, Twelve Months or Longer, Fair Value	27,674	31,287
Available-for-sale Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss	(139)	(102)
Available-for-sale Securities, Continuous Unrealized Loss Position, Fair Value	50,567	46,781
Available-for-sale Securities, Continuous Unrealized Loss Position, Accumulated Loss	(323)	(145)
Available-for-sale Securities, Debt Maturities, Next Rolling Twelve Months, Amortized Cost Basis	20,025	18,274
Available-for-sale Securities, Debt Maturities, Next Rolling Twelve Months, Fair Value	19,962	18,244
Available-for-sale Securities, Debt Maturities, Rolling Year Two Through Five, Amortized Cost Basis	30,865	32,184
Available-for-sale Securities, Debt Maturities, Rolling Year Two Through Five, Fair Value	30,605	32,071
Certificates of deposit
Schedule of Available-for-sale Securities [Line Items]
Amortized cost	221	240
Gross unrealized gains	0	0
Gross unrealized losses	(1)	0
Fair value	220	240
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than Twelve Months, Fair Value	220	240
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss	(1)	0
Available-for-sale Securities, Continuous Unrealized Loss Position, Twelve Months or Longer, Fair Value	0	0
Available-for-sale Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss	0	0
Available-for-sale Securities, Continuous Unrealized Loss Position, Fair Value	220	240
Available-for-sale Securities, Continuous Unrealized Loss Position, Accumulated Loss	(1)	0
Corporate notes/bonds
Schedule of Available-for-sale Securities [Line Items]
Amortized cost	48,268	47,811
Gross unrealized gains	0	2
Gross unrealized losses	(311)	(140)
Fair value	47,957	47,673
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than Twelve Months, Fair Value	22,673	15,254
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss	(183)	(43)
Available-for-sale Securities, Continuous Unrealized Loss Position, Twelve Months or Longer, Fair Value	25,284	28,885
Available-for-sale Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss	(128)	(97)
Available-for-sale Securities, Continuous Unrealized Loss Position, Fair Value	47,957	44,139
Available-for-sale Securities, Continuous Unrealized Loss Position, Accumulated Loss	(311)	(140)
U.S. government agency securities
Schedule of Available-for-sale Securities [Line Items]
Amortized cost	2,401	2,407
Gross unrealized gains	0	0
Gross unrealized losses	(11)	(5)
Fair value	2,390	2,402
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	2,390	2,402
Available-for-sale Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss	(11)	(5)
Available-for-sale Securities, Continuous Unrealized Loss Position, Fair Value	2,390	2,402
Available-for-sale Securities, Continuous Unrealized Loss Position, Accumulated Loss	$ (11)	$ (5)