Fair Value Measurements - Cash Equivalents (Details) - Fair Value, Measurements, Recurring - USD ($) $ in Thousands	Sep. 30, 2018	Sep. 30, 2017
Cash equivalents
Assets:
Cash and cash equivalents	$ 50,572	$ 45
Quoted Prices in Active Markets for Identical Assets (Level 1) | Cash equivalents
Assets:
Cash and cash equivalents	50,572	42
Quoted Prices in Active Markets for Identical Assets (Level 1) | Money market funds
Assets:
Cash and cash equivalents	50,600
Quoted Prices in Active Markets for Identical Assets (Level 1) | Money market funds | Maximum
Assets:
Cash and cash equivalents	$ 50,600	100
Significant Other Observable Inputs (Level 2) | Cash equivalents
Assets:
Cash and cash equivalents		3
Significant Other Observable Inputs (Level 2) | Bank certificates of deposits | Maximum
Assets:
Cash and cash equivalents		$ 100