Short-Term and Long-Term Investments (Schedule of Short-Term and Long-Term Investments) (Details) - USD ($) $ in Thousands	Jun. 30, 2017	Sep. 30, 2016
Schedule of Investments [Line Items]
Cost or Amortized Cost	$ 558,617
Fair Value	557,923
Long Term Investments [Member]
Schedule of Investments [Line Items]
Cost or Amortized Cost	235,060	$ 276,156
Gross Unrealized Gains	93	383
Gross Unrealized Losses	(566)	(164)
Fair Value	234,587	276,375
Long Term Investments [Member] | Corporate bonds and notes
Schedule of Investments [Line Items]
Cost or Amortized Cost	130,725	131,273
Gross Unrealized Gains	32	249
Gross Unrealized Losses	(273)	(95)
Fair Value	130,484	131,427
Long Term Investments [Member] | Municipal bonds and notes
Schedule of Investments [Line Items]
Cost or Amortized Cost	30,926	27,017
Gross Unrealized Gains	60	46
Gross Unrealized Losses	(22)	(13)
Fair Value	30,964	27,050
Long Term Investments [Member] | U.S. government securities
Schedule of Investments [Line Items]
Cost or Amortized Cost	12,964	26,989
Gross Unrealized Gains	0	36
Gross Unrealized Losses	(22)	(2)
Fair Value	12,942	27,023
Long Term Investments [Member] | U.S. government agency securities
Schedule of Investments [Line Items]
Cost or Amortized Cost	60,445	90,877
Gross Unrealized Gains	1	52
Gross Unrealized Losses	(249)	(54)
Fair Value	60,197	90,875
Short-term Investments [Member]
Schedule of Investments [Line Items]
Cost or Amortized Cost	323,557	367,920
Gross Unrealized Gains	23	76
Gross Unrealized Losses	(244)	(172)
Fair Value	323,336	367,824
Short-term Investments [Member] | Certificates of Deposit [Member]
Schedule of Investments [Line Items]
Cost or Amortized Cost	1,999
Gross Unrealized Gains	0
Gross Unrealized Losses	0
Fair Value	1,999
Short-term Investments [Member] | Corporate bonds and notes
Schedule of Investments [Line Items]
Cost or Amortized Cost	150,662	223,448
Gross Unrealized Gains	12	38
Gross Unrealized Losses	(101)	(128)
Fair Value	150,573	223,358
Short-term Investments [Member] | Municipal bonds and notes
Schedule of Investments [Line Items]
Cost or Amortized Cost	62,191	53,657
Gross Unrealized Gains	11	1
Gross Unrealized Losses	(9)	(37)
Fair Value	62,193	53,621
Short-term Investments [Member] | U.S. government securities
Schedule of Investments [Line Items]
Cost or Amortized Cost	101,035	72,497
Gross Unrealized Gains	0	31
Gross Unrealized Losses	(117)	(7)
Fair Value	100,918	72,521
Short-term Investments [Member] | U.S. government agency securities
Schedule of Investments [Line Items]
Cost or Amortized Cost	7,670	18,318
Gross Unrealized Gains	0	6
Gross Unrealized Losses	(17)	0
Fair Value	$ 7,653	$ 18,324