Income Taxes - Summary of Federal Income Tax Rate and Effective Income Tax Rate (Details) - USD ($) $ in Thousands	12 Months Ended
Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2018
Reconciliation, Amount
Statutory federal income tax rate	$ 28,288	$ 4,263	$ 7,021
Change in valuation allowance	(16,489)	66,417	(181,047)
Transition impact of U.S. Tax Act	0	0	126,122
Federal rate change	0	0	69,648
State income taxes, net of federal tax benefit	(2,998)	607	2,401
Federal research and development credits	(5,483)	(3,731)	(3,058)
Uncertain tax positions	3,072	2,611	(4,646)
Foreign rate differences	(22,074)	(26,952)	(38,743)
Foreign tax on U.S. provision	4,523	6,547	2,736
Excess tax benefits from restricted stock	(1,743)	(5,940)	(11,641)
Audits and settlements	0	51	2,352
U.S. permanent items	6,590	2,483	5,408
BEAT	(1,759)	1,759	0
GILTI, net of foreign tax credits	14,899	6,170	0
Foreign-Derived Intangible Income (FDII)	(2,461)	(6,409)	0
Other, net	(354)	(116)	116
Total provision (benefit) for income taxes	$ 4,011	$ 47,760	$ (23,331)
Reconciliation, Percent
Statutory federal income tax rate	21.00%	21.00%	25.00%
Change in valuation allowance	(12.00%)	327.00%	(632.00%)
Transition impact of U.S. Tax Act	0.00%	0.00%	440.00%
Federal rate change	0.00%	0.00%	243.00%
State income taxes, net of federal tax benefit	(2.00%)	3.00%	8.00%
Federal research and development credits	(4.00%)	(18.00%)	(11.00%)
Uncertain tax positions	2.00%	13.00%	(16.00%)
Foreign rate differences	(16.00%)	(133.00%)	(135.00%)
Foreign tax on U.S. provision	3.00%	32.00%	10.00%
Excess tax benefits from restricted stock	(1.00%)	(29.00%)	(41.00%)
Audits and settlements	0.00%	0.00%	8.00%
U.S. permanent items	5.00%	12.00%	19.00%
BEAT	(1.00%)	9.00%	0.00%
GILTI, net of foreign tax credits	11.00%	31.00%	0.00%
Foreign-Derived Intangible Income (FDII)	(2.00%)	(32.00%)	0.00%
Other, net	(1.00%)	(1.00%)	1.00%
Provision (benefit) for income taxes	3.00%	235.00%	(81.00%)

Effective Income Tax Rate Reconciliation, Excess Tax Benefits from Restricted Stock, Amount

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