Short-Term and Long-Term Investments (Schedule of Investments That Have Been in a Continuous Unrealized Loss Position) (Details) - USD ($) $ in Thousands	Jun. 30, 2019	Sep. 30, 2018
Fair Value
Less Than 12 Months	$ 67,169	$ 718,857
12 Months or Greater	194,212	232,826
Total	261,381	951,683
Gross Unrealized Losses
Less Than 12 Months	(70)	(2,314)
12 Months or Greater	(405)	(2,245)
Total	(475)	(4,559)
Corporate Bonds and Notes [Member]
Fair Value
Less Than 12 Months	66,064	543,729
12 Months or Greater	165,970	152,097
Total	232,034	695,826
Gross Unrealized Losses
Less Than 12 Months	(70)	(1,800)
12 Months or Greater	(352)	(1,835)
Total	(422)	(3,635)
Municipal Bonds and Notes [Member]
Fair Value
Less Than 12 Months	1,105	26,846
12 Months or Greater	10,786	14,363
Total	11,891	41,209
Gross Unrealized Losses
Less Than 12 Months	0	(123)
12 Months or Greater	(23)	(124)
Total	(23)	(247)
U.S. Government Securities [Member]
Fair Value
Less Than 12 Months	0	103,470
12 Months or Greater	12,964	29,379
Total	12,964	132,849
Gross Unrealized Losses
Less Than 12 Months	0	(281)
12 Months or Greater	(23)	(109)
Total	(23)	(390)
U.S. Government Agency Securities [Member]
Fair Value
Less Than 12 Months	0	44,812
12 Months or Greater	4,492	36,987
Total	4,492	81,799
Gross Unrealized Losses
Less Than 12 Months	0	(110)
12 Months or Greater	(7)	(177)
Total	$ (7)	$ (287)