Marketable Securities (Schedule of Fair Value and Gross Unrealized Losses) (Details) - USD ($) $ in Thousands	Jun. 30, 2018	Sep. 30, 2017
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than Twelve Months, Fair Value	$ 31,735	$ 15,494
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss	(360)	(43)
Available-for-sale Securities, Continuous Unrealized Loss Position, Twelve Months or Longer, Fair Value	22,437	31,287
Available-for-sale Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss	(163)	(102)
Available-for-sale Securities, Continuous Unrealized Loss Position, Fair Value	54,172	46,781
Available-for-sale Securities, Continuous Unrealized Loss Position, Accumulated Loss	(523)	(145)
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	219	240
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss	(2)	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	(2)	0
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	31,516	15,254
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss	(358)	(43)
Available-for-sale Securities, Continuous Unrealized Loss Position, Twelve Months or Longer, Fair Value	21,446	28,885
Available-for-sale Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss	(154)	(97)
Available-for-sale Securities, Continuous Unrealized Loss Position, Fair Value	52,962	44,139
Available-for-sale Securities, Continuous Unrealized Loss Position, Accumulated Loss	(512)	(140)
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	991	2,402
Available-for-sale Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss	(9)	(5)
Available-for-sale Securities, Continuous Unrealized Loss Position, Fair Value	991	2,402
Available-for-sale Securities, Continuous Unrealized Loss Position, Accumulated Loss	$ (9)	$ (5)