Short-Term and Long-Term Investments - Schedule of Investments That Have Been in a Continuous Unrealized Loss Position (Details) - USD ($) $ in Thousands	Jun. 30, 2020	Sep. 30, 2019
Fair Value
Less than 12 months	$ 233,790	$ 296,652
12 months or greater	5,005	127,404
Total	238,795	424,056
Gross Unrealized Losses
Less than 12 months	(135)	(468)
12 months or greater	(5)	(120)
Total	(140)	(588)
Corporate Bonds and Notes
Fair Value
Less than 12 months	71,355	237,747
12 months or greater	5,005	109,613
Total	76,360	347,360
Gross Unrealized Losses
Less than 12 months	(92)	(434)
12 months or greater	(5)	(104)
Total	(97)	(538)
Municipal Bonds and Notes
Fair Value
Less than 12 months		864
12 months or greater		7,800
Total		8,664
Gross Unrealized Losses
Less than 12 months		0
12 months or greater		(11)
Total		(11)
U.S. Government Securities
Fair Value
Less than 12 months	152,646	27,095
12 months or greater	0	9,991
Total	152,646	37,086
Gross Unrealized Losses
Less than 12 months	(37)	(12)
12 months or greater	0	(5)
Total	(37)	(17)
U.S. Government Agency Securities
Fair Value
Less than 12 months	9,789	30,946
12 months or greater	0	0
Total	9,789	30,946
Gross Unrealized Losses
Less than 12 months	(6)	(22)
12 months or greater	0	0
Total	$ (6)	$ (22)