Marketable Securities - Cash Equivalents (Details) - Fair Value, Measurements, Recurring - USD ($) $ in Millions	Jun. 30, 2018	Sep. 30, 2017
Quoted Prices in Active Markets for Identical Assets (Level 1) | Money Market Funds
Assets:
Cash equivalents	$ 50.3
Quoted Prices in Active Markets for Identical Assets (Level 1) | Money Market Funds | Maximum
Assets:
Cash equivalents		$ 0.1
Significant Other Observable Inputs (Level 2)
Assets:
Cash equivalents	$ 0.5
Significant Other Observable Inputs (Level 2) | Maximum
Assets:
Cash equivalents		$ 0.1