Marketable Securities (Fair Value and Gross Unrealized Losses for AFS) (Details) - USD ($) $ in Thousands	Jun. 30, 2017	Sep. 30, 2016
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	$ 12,463	$ 48,397
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss	(14)	(53)
Fair value of available-for-sale securities in continuous unrealized loss position for twelve months or longer	0	4,102
Available-for-sale Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss	0	(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	10,213	24,454
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss	(13)	(33)
Fair value of available-for-sale securities in continuous unrealized loss position for twelve months or longer	0	4,102
Available-for-sale Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss	0	(1)
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	1,999	23,943
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss	(1)	(20)
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	251
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss	0
Fair value of available-for-sale securities in continuous unrealized loss position for twelve months or longer	0
Available-for-sale Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss	$ 0

Amount of accumulated unrealized loss on investments in debt and equity securities classified as available-for-sale that have been in a continuous loss position for twelve months or longer.

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 50
-Paragraph 6
-Subparagraph (a)(2)
-URI http://asc.fasb.org/extlink&oid=82897076&loc=d3e27290-111563

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
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Reference 1: http://www.xbrl.org/2003/role/presentationRef
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Reference 1: http://www.xbrl.org/2003/role/presentationRef
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