Marketable Securities (Fair Value and Gross Unrealized Losses for AFS) (Details) - USD ($) $ in Thousands	Mar. 31, 2016	Sep. 30, 2015
Schedule of Available-for-sale Securities [Line Items]
Fair value of available-for-sale securities in continuous unrealized loss position for less than twelve months	$ 40,887	$ 47,054
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss	(49)	(62)
Fair value of available-for-sale securities in continuous unrealized loss position for twelve months or longer	3,002	499
Available-for-sale Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss	(1)	(1)
Corporate Bonds
Schedule of Available-for-sale Securities [Line Items]
Fair value of available-for-sale securities in continuous unrealized loss position for less than twelve months	19,843	33,664
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss	(30)	(52)
Fair value of available-for-sale securities in continuous unrealized loss position for twelve months or longer	3,002	0
Available-for-sale Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss	(1)	0
Commercial Paper
Schedule of Available-for-sale Securities [Line Items]
Fair value of available-for-sale securities in continuous unrealized loss position for less than twelve months	17,960	5,987
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss	(14)	(1)
Fair value of available-for-sale securities in continuous unrealized loss position for twelve months or longer	0	0
Available-for-sale Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss	0	0
Certificates of Deposit
Schedule of Available-for-sale Securities [Line Items]
Fair value of available-for-sale securities in continuous unrealized loss position for less than twelve months		4,244
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss		(6)
Fair value of available-for-sale securities in continuous unrealized loss position for twelve months or longer		499
Available-for-sale Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss		(1)
Government municipal bonds
Schedule of Available-for-sale Securities [Line Items]
Fair value of available-for-sale securities in continuous unrealized loss position for less than twelve months	3,084	3,159
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss	(5)	(3)
Fair value of available-for-sale securities in continuous unrealized loss position for twelve months or longer	0	0
Available-for-sale Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss	$ 0	$ 0