Marketable Securities (Details) - USD ($) $ in Thousands	Mar. 31, 2018	Sep. 30, 2017
Schedule of Available-for-sale Securities [Line Items]
Amortized cost	$ 55,853	$ 50,458
Gross unrealized gains	0	2
Gross unrealized losses	(589)	(145)
Fair value	55,264	50,315
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than Twelve Months, Fair Value	30,931	15,494
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss	(414)	(43)
Available-for-sale Securities, Continuous Unrealized Loss Position, Twelve Months or Longer, Fair Value	24,333	31,287
Available-for-sale Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss	(175)	(102)
Available-for-sale Securities, Continuous Unrealized Loss Position, Fair Value	55,264	46,781
Available-for-sale Securities, Continuous Unrealized Loss Position, Accumulated Loss	(589)	(145)
Available-for-sale Securities, Debt Maturities, Next Rolling Twelve Months, Amortized Cost Basis	22,772	18,274
Available-for-sale Securities, Debt Maturities, Next Rolling Twelve Months, Fair Value	22,636	18,244
Available-for-sale Securities, Debt Maturities, Rolling Year Two Through Five, Amortized Cost Basis	33,081	32,184
Available-for-sale Securities, Debt Maturities, Rolling Year Two Through Five, Fair Value	32,628	32,071
Available-for-sale Debt Securities, Amortized Cost Basis	55,853	50,458
Available-for-sale Securities, Debt Securities	55,264	50,315
Certificates of deposit
Schedule of Available-for-sale Securities [Line Items]
Amortized cost	220	240
Gross unrealized gains	0	0
Gross unrealized losses	(1)	0
Fair value	219	240
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than Twelve Months, Fair Value	219	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	219	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	53,225	47,811
Gross unrealized gains	0	2
Gross unrealized losses	(575)	(140)
Fair value	52,650	47,673
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than Twelve Months, Fair Value	30,712	15,254
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss	(413)	(43)
Available-for-sale Securities, Continuous Unrealized Loss Position, Twelve Months or Longer, Fair Value	21,938	28,885
Available-for-sale Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss	(162)	(97)
Available-for-sale Securities, Continuous Unrealized Loss Position, Fair Value	52,650	44,139
Available-for-sale Securities, Continuous Unrealized Loss Position, Accumulated Loss	(575)	(140)
U.S. government agency securities
Schedule of Available-for-sale Securities [Line Items]
Amortized cost	2,408	2,407
Gross unrealized gains	0	0
Gross unrealized losses	(13)	(5)
Fair value	2,395	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	2,395	2,402
Available-for-sale Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss	(13)	(5)
Available-for-sale Securities, Continuous Unrealized Loss Position, Fair Value	2,395	2,402
Available-for-sale Securities, Continuous Unrealized Loss Position, Accumulated Loss	$ (13)	$ (5)