Fair Value Measurements - Financial Assets and Liabilities Measured at Fair Value on a Recurring Basis (Details) - USD ($) $ in Thousands	Sep. 30, 2023	Sep. 30, 2022
Assets, Fair Value Disclosure [Abstract]
Available-for-sale securities	$ 450,211	$ 1,263,784
Derivative Asset, Statement of Financial Position	Prepaid expenses and other current assets	Prepaid expenses and other current assets
Fair Value, Recurring
Assets, Fair Value Disclosure [Abstract]
Available-for-sale securities	$ 450,211	$ 1,263,782
Foreign exchange contracts	44	634
Net investment hedge	13,036	124,789
Total Assets	989,243	1,764,009
Liabilities, Fair Value Disclosure [Abstract]
Foreign exchange contracts	421	230
Total Liabilities	421	230
Fair Value, Recurring | Cash Equivalents
Assets, Fair Value Disclosure [Abstract]
Cash equivalents	525,952	374,804
Fair Value, Recurring | Fair Value, Inputs, Level 1
Assets, Fair Value Disclosure [Abstract]
Available-for-sale securities	85,949	651,800
Total Assets	611,901	1,025,855
Fair Value, Recurring | Fair Value, Inputs, Level 1 | Cash Equivalents
Assets, Fair Value Disclosure [Abstract]
Cash equivalents	525,952	374,055
Fair Value, Recurring | Fair Value, Inputs, Level 2
Assets, Fair Value Disclosure [Abstract]
Available-for-sale securities	364,262	611,982
Foreign exchange contracts	44	634
Net investment hedge	13,036	124,789
Total Assets	377,342	738,154
Liabilities, Fair Value Disclosure [Abstract]
Foreign exchange contracts	421	230
Total Liabilities	$ 421	230
Fair Value, Recurring | Fair Value, Inputs, Level 2 | Cash Equivalents
Assets, Fair Value Disclosure [Abstract]
Cash equivalents		$ 749