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Marketable Securities - Cash Equivalents (Details) - Fair Value, Measurements, Recurring - USD ($)
$ in Millions
Mar. 31, 2020
Sep. 30, 2019
Quoted Prices in Active Markets for Identical Assets (Level 1) | Money market funds    
Assets:    
Cash equivalents   $ 6.2
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   $ 10.0