Investments & Fair Value Measurements (Schedule Of Financial Assets and Liabilities Carried At Fair Value) (Details) - USD ($) $ in Thousands	Jun. 30, 2017	Sep. 30, 2016
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Cash and Cash Equivalents, Fair Value Disclosure	$ 592,616	$ 516,112
Available-for-sale Debt Securities, Amortized Cost Basis	560,325	525,904
Available-for-sale Securities, Debt Securities	559,474	526,420
Investment Owned, Unrecognized Unrealized Depreciation	(1,311)	(375)
Cash Cash Equivalents And Investments Total	1,143,601	1,031,645
Financial assets carried at fair value		516,112
Restricted Cash and Cash Equivalents	6,187	3,645
Long-term investments	303,320	393,904
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than Twelve Months, Fair Value	321,793	223,583
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss	(1,299)	(370)
Available-for-sale Securities, Continuous Unrealized Loss Position, Twelve Months or Longer, Fair Value	2,399	5,615
Available-for-sale Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss	(12)	(5)
Available-for-sale Securities, Debt Maturities, Next Twelve Months, Amortized Cost Basis	251,614	135,886
Available-for-sale Securities, Debt Maturities, Next Twelve Months, Fair Value	251,484	135,884
Available For Sale Securities Debt Maturities After One Through Two Years Amortized Cost	191,471	225,679
Available For Sale Securities Debt Maturities After One Through Two Years Fair Value	191,239	225,953
Available For Sale Securities Debt Maturities After Two Through Three Years Amortized Cost	117,240	164,339
Available For Sale Securities Debt Maturities After Two Through Three Years Fair Value	116,751	164,583
Investment Owned, at Cost	1,144,108	1,030,769
Investment Owned, Unrecognized Unrealized Appreciation	804	1,251
Level 1 [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Cash and Cash Equivalents, Fair Value Disclosure	14,719	10,910
Financial assets carried at fair value	61,176	70,846
Level 2 [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Cash and Cash Equivalents, Fair Value Disclosure	265	3,339
Financial assets carried at fair value	500,791	455,934
Cash [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Cash and Cash Equivalents, Fair Value Disclosure	577,632	501,863
Money Market Funds [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Cash and Cash Equivalents, Fair Value Disclosure	14,719	10,910
Money Market Funds [Member] | Level 1 [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Cash and Cash Equivalents, Fair Value Disclosure	14,719	10,910
Certificates of Deposit [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Cash and Cash Equivalents, Fair Value Disclosure	150
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than Twelve Months, Fair Value	20,169	0
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss	(7)	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
Certificates of Deposit [Member] | Level 2 [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Cash and Cash Equivalents, Fair Value Disclosure	150
U.S. Agency Securities [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than Twelve Months, Fair Value	27,292	22,988
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss	(225)	(38)
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
Government Bonds [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than Twelve Months, Fair Value	18,819	0
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss	(187)	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
Commercial Paper [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than Twelve Months, Fair Value	3,999	11,479
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss	(2)	(10)
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
Corporate Bonds [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Cash and Cash Equivalents, Fair Value Disclosure		2,240
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than Twelve Months, Fair Value	217,429	153,491
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss	(815)	(280)
Available-for-sale Securities, Continuous Unrealized Loss Position, Twelve Months or Longer, Fair Value	2,399	1,000
Available-for-sale Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss	(12)	0
Corporate Bonds [Member] | Level 1 [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Cash and Cash Equivalents, Fair Value Disclosure		2,240
Municipal Bonds [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Available-for-sale Debt Securities, Amortized Cost Basis	115
Available-for-sale Debt Securities, Accumulated Gross Unrealized Gain, before Tax	0
Available-for-sale Debt Securities, Accumulated Gross Unrealized Loss, before Tax	0
Available-for-sale Securities, Debt Securities	115
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than Twelve Months, Fair Value	34,085	35,625
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss	(63)	(42)
Available-for-sale Securities, Continuous Unrealized Loss Position, Twelve Months or Longer, Fair Value	0	4,615
Available-for-sale Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss	0	(5)
Municipal Bonds [Member] | Level 2 [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Investments, Fair Value Disclosure	115
Other Long-term Investments [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Investments, Noncurrent, Fair Value Disclosure	500	500
Cost Method Investment, Fair Value Measurement Adjustment	300	400
Investments Held In Supplemental Retirement Plan [Member] | Level 1 [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Financial assets carried at fair value	2,918	2,638
Financial liabilities carried at fair value	2,918	2,638
Cost Method Investment 1 [Member] | Other Long-term Investments [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Long-term investments	3,000	2,000
Cost Method Investment 2 [Member] | Other Long-term Investments [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Long-term investments	500	500
Short-term Investments [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Available-for-sale Debt Securities, Amortized Cost Basis	247,827	121,637
Available-for-sale Debt Securities, Accumulated Gross Unrealized Gain, before Tax	95	31
Available-for-sale Debt Securities, Accumulated Gross Unrealized Loss, before Tax	(257)	(39)
Available-for-sale Securities, Debt Securities	247,665	121,629
Short-term Investments [Member] | Level 1 [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Investments, Fair Value Disclosure	9,716	0
Short-term Investments [Member] | Level 2 [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Investments, Fair Value Disclosure	237,949	121,629
Short-term Investments [Member] | Certificates of Deposit [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Available-for-sale Securities, Amortized Cost Basis	15,559	13,912
Available-for-sale Securities, Accumulated Gross Unrealized Gain, before Tax	14	6
Available-for-sale Securities, Accumulated Gross Unrealized Loss, before Tax	0	0
Available-for-sale Securities	15,573	13,918
Short-term Investments [Member] | Certificates of Deposit [Member] | Level 2 [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Investments, Fair Value Disclosure	15,573	13,918
Short-term Investments [Member] | U.S. Agency Securities [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Available-for-sale Debt Securities, Amortized Cost Basis	9,214
Available-for-sale Debt Securities, Accumulated Gross Unrealized Gain, before Tax	0
Available-for-sale Debt Securities, Accumulated Gross Unrealized Loss, before Tax	(30)
Available-for-sale Securities, Debt Securities	9,184
Short-term Investments [Member] | U.S. Agency Securities [Member] | Level 1 [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Investments, Fair Value Disclosure	9,184
Short-term Investments [Member] | Government Bonds [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Available-for-sale Securities, Amortized Cost Basis	535
Available-for-sale Securities, Accumulated Gross Unrealized Gain, before Tax	0
Available-for-sale Securities, Accumulated Gross Unrealized Loss, before Tax	(3)
Available-for-sale Securities	532
Short-term Investments [Member] | Government Bonds [Member] | Level 1 [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Investments, Noncurrent, Fair Value Disclosure	532
Short-term Investments [Member] | Commercial Paper [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Cash and Cash Equivalents, Fair Value Disclosure		1,099
Available-for-sale Debt Securities, Amortized Cost Basis	20,596	19,629
Available-for-sale Debt Securities, Accumulated Gross Unrealized Gain, before Tax	2	1
Available-for-sale Debt Securities, Accumulated Gross Unrealized Loss, before Tax	(2)	(10)
Available-for-sale Securities, Debt Securities	20,596	19,620
Short-term Investments [Member] | Commercial Paper [Member] | Level 2 [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Cash and Cash Equivalents, Fair Value Disclosure		1,099
Investments, Fair Value Disclosure	20,596	19,620
Short-term Investments [Member] | Corporate Bonds [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Available-for-sale Debt Securities, Amortized Cost Basis	177,039	63,762
Available-for-sale Debt Securities, Accumulated Gross Unrealized Gain, before Tax	67	24
Available-for-sale Debt Securities, Accumulated Gross Unrealized Loss, before Tax	(216)	(14)
Available-for-sale Securities, Debt Securities	176,890	63,772
Short-term Investments [Member] | Corporate Bonds [Member] | Level 2 [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Investments, Fair Value Disclosure	176,890	63,772
Short-term Investments [Member] | Municipal Bonds [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Available-for-sale Debt Securities, Amortized Cost Basis	24,884	24,334
Available-for-sale Debt Securities, Accumulated Gross Unrealized Gain, before Tax	12	0
Available-for-sale Debt Securities, Accumulated Gross Unrealized Loss, before Tax	(6)	(15)
Available-for-sale Securities, Debt Securities	24,890	24,319
Short-term Investments [Member] | Municipal Bonds [Member] | Level 2 [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Investments, Fair Value Disclosure	24,890	24,319
Long-term Investments [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Available-for-sale Securities, Amortized Cost Basis	303,665	393,020
Available-for-sale Securities, Accumulated Gross Unrealized Gain, before Tax	709	1,220
Available-for-sale Securities, Accumulated Gross Unrealized Loss, before Tax	(1,054)	(336)
Available-for-sale Securities	303,320	393,904
Long-term Investments [Member] | Level 1 [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Investments, Noncurrent, Fair Value Disclosure	36,741	59,936
Long-term Investments [Member] | Level 2 [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Investments, Noncurrent, Fair Value Disclosure	262,577	330,966
Long-term Investments [Member] | Certificates of Deposit [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Available-for-sale Securities, Amortized Cost Basis	24,046	4,500
Available-for-sale Securities, Accumulated Gross Unrealized Gain, before Tax	8	10
Available-for-sale Securities, Accumulated Gross Unrealized Loss, before Tax	(7)	0
Available-for-sale Securities	24,047	4,510
Long-term Investments [Member] | Certificates of Deposit [Member] | Level 2 [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Investments, Noncurrent, Fair Value Disclosure	24,047	4,510
Long-term Investments [Member] | U.S. Agency Securities [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Available-for-sale Securities, Amortized Cost Basis	18,304	27,536
Available-for-sale Securities, Accumulated Gross Unrealized Gain, before Tax	0	24
Available-for-sale Securities, Accumulated Gross Unrealized Loss, before Tax	(195)	(26)
Available-for-sale Securities	18,109	27,534
Long-term Investments [Member] | U.S. Agency Securities [Member] | Level 1 [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Investments, Noncurrent, Fair Value Disclosure	18,109	27,534
Long-term Investments [Member] | Government Bonds [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Available-for-sale Securities, Amortized Cost Basis	18,471	31,971
Available-for-sale Securities, Accumulated Gross Unrealized Gain, before Tax	0	77
Available-for-sale Securities, Accumulated Gross Unrealized Loss, before Tax	(184)	(12)
Available-for-sale Securities	18,287	32,036
Long-term Investments [Member] | Government Bonds [Member] | Level 1 [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Investments, Noncurrent, Fair Value Disclosure	18,287	32,036
Long-term Investments [Member] | Corporate Bonds [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Available-for-sale Securities, Amortized Cost Basis	201,043	295,921
Available-for-sale Securities, Accumulated Gross Unrealized Gain, before Tax	322	715
Available-for-sale Securities, Accumulated Gross Unrealized Loss, before Tax	(611)	(266)
Available-for-sale Securities	200,754	296,370
Long-term Investments [Member] | Corporate Bonds [Member] | Level 2 [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Investments, Noncurrent, Fair Value Disclosure	200,754	296,370
Long-term Investments [Member] | Municipal Bonds [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Available-for-sale Securities, Amortized Cost Basis	37,799	30,090
Available-for-sale Securities, Accumulated Gross Unrealized Gain, before Tax	34	28
Available-for-sale Securities, Accumulated Gross Unrealized Loss, before Tax	(57)	(32)
Available-for-sale Securities	37,776	30,086
Long-term Investments [Member] | Municipal Bonds [Member] | Level 2 [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Investments, Noncurrent, Fair Value Disclosure	37,776	30,086
Long-term Investments [Member] | Other Long-term Investments [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Available-for-sale Securities, Amortized Cost Basis	4,002	3,002
Available-for-sale Securities, Accumulated Gross Unrealized Gain, before Tax	345	366
Available-for-sale Securities, Accumulated Gross Unrealized Loss, before Tax	0	0
Available-for-sale Securities	4,347	3,368
Long-term Investments [Member] | Other Long-term Investments [Member] | Level 1 [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Investments, Noncurrent, Fair Value Disclosure	$ 345	$ 366