Fair Value Measurements - Assets and Liabilities Measured at Fair Value on Recurring Basis (Details) - USD ($) $ in Thousands	Sep. 30, 2020	Sep. 30, 2019
Assets:
Available-for-sale securities	$ 3,168	$ 36,969
Fair Value, Measurements, Recurring
Assets:
Available-for-sale securities	3,168	36,969
Foreign exchange contracts	370	17
Total Assets	3,588	53,150
Liabilities:
Foreign exchange contracts	238	340
Total Liabilities	238	340
Fair Value, Measurements, Recurring | Cash equivalents
Assets:
Cash equivalents	50	16,164
Quoted Prices in Active Markets for Identical Assets (Level 1) | Fair Value, Measurements, Recurring
Assets:
Total Assets		6,188
Quoted Prices in Active Markets for Identical Assets (Level 1) | Fair Value, Measurements, Recurring | Cash equivalents
Assets:
Cash equivalents		6,188
Significant Other Observable Inputs (Level 2) | Fair Value, Measurements, Recurring
Assets:
Cash equivalents	100	10,000
Available-for-sale securities	3,168	36,969
Foreign exchange contracts	370	17
Total Assets	3,588	46,962
Liabilities:
Foreign exchange contracts	238	340
Total Liabilities	238	340
Significant Other Observable Inputs (Level 2) | Fair Value, Measurements, Recurring | Cash equivalents
Assets:
Cash equivalents	$ 50	$ 9,976