Marketable Securities - Cash Equivalents (Details) - Fair Value, Measurements, Recurring - USD ($) $ in Millions	Dec. 31, 2018	Sep. 30, 2018
Quoted Prices in Active Markets for Identical Assets (Level 1) | Money market funds
Assets:
Cash and cash equivalents		$ 50.6
Quoted Prices in Active Markets for Identical Assets (Level 1) | Money market funds | Maximum
Assets:
Cash and cash equivalents	$ 0.1
Significant Other Observable Inputs (Level 2) | Bank certificates of deposits | Maximum
Assets:
Cash and cash equivalents	$ 0.1