Short-Term and Long-Term Investments - Schedule of Short-Term and Long-Term Investments (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Jun. 30, 2020	Jun. 30, 2019	Jun. 30, 2020	Jun. 30, 2019	Sep. 30, 2019
Schedule of Investments [Line Items]
Interest income from investments	$ 2,200	$ 5,800	$ 11,100	$ 20,200
Short-Term Investments
Schedule of Investments [Line Items]
Cost or amortized cost	378,387		378,387		$ 372,745
Gross unrealized gains	970		970		435
Gross unrealized losses	(54)		(54)		(117)
Fair value	379,303		379,303		373,063
Short-Term Investments | Certificates of Deposit
Schedule of Investments [Line Items]
Cost or amortized cost					249
Gross unrealized gains					0
Gross unrealized losses					0
Fair value					249
Short-Term Investments | Corporate Bonds and Notes
Schedule of Investments [Line Items]
Cost or amortized cost	202,968		202,968		259,242
Gross unrealized gains	900		900		402
Gross unrealized losses	(17)		(17)		(97)
Fair value	203,851		203,851		259,547
Short-Term Investments | Municipal Bonds and Notes
Schedule of Investments [Line Items]
Cost or amortized cost	7,018		7,018		12,128
Gross unrealized gains	22		22		6
Gross unrealized losses	0		0		(5)
Fair value	7,040		7,040		12,129
Short-Term Investments | U.S. Government Securities
Schedule of Investments [Line Items]
Cost or amortized cost	161,414		161,414		78,988
Gross unrealized gains	38		38		18
Gross unrealized losses	(35)		(35)		(14)
Fair value	161,417		161,417		78,992
Short-Term Investments | U.S. Government Agency Securities
Schedule of Investments [Line Items]
Cost or amortized cost	6,987		6,987		22,138
Gross unrealized gains	10		10		9
Gross unrealized losses	(2)		(2)		(1)
Fair value	6,995		6,995		22,146
Long-Term Investments
Schedule of Investments [Line Items]
Cost or amortized cost	111,879		111,879		358,394
Gross unrealized gains	1,135		1,135		479
Gross unrealized losses	(86)		(86)		(471)
Fair value	112,928		112,928		358,402
Long-Term Investments | Corporate Bonds and Notes
Schedule of Investments [Line Items]
Cost or amortized cost	96,645		96,645		298,888
Gross unrealized gains	1,112		1,112		469
Gross unrealized losses	(80)		(80)		(441)
Fair value	97,677		97,677		298,916
Long-Term Investments | Municipal Bonds and Notes
Schedule of Investments [Line Items]
Cost or amortized cost					2,528
Gross unrealized gains					2
Gross unrealized losses					(6)
Fair value					2,524
Long-Term Investments | U.S. Government Securities
Schedule of Investments [Line Items]
Cost or amortized cost	3,984		3,984		5,515
Gross unrealized gains	0		0		3
Gross unrealized losses	(2)		(2)		(3)
Fair value	3,982		3,982		5,515
Long-Term Investments | U.S. Government Agency Securities
Schedule of Investments [Line Items]
Cost or amortized cost	11,250		11,250		51,463
Gross unrealized gains	23		23		5
Gross unrealized losses	(4)		(4)		(21)
Fair value	$ 11,269		$ 11,269		$ 51,447