Quarterly report pursuant to Section 13 or 15(d)

Commitments and Contingencies - Operating Leases (Details)

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Commitments and Contingencies - Operating Leases (Details) - Operating Lease, March 2015
$ in Millions
1 Months Ended 3 Months Ended 9 Months Ended
Apr. 30, 2018
USD ($)
Mar. 31, 2015
USD ($)
ft²
Sep. 30, 2018
USD ($)
Sep. 30, 2017
USD ($)
Sep. 30, 2018
USD ($)
Sep. 30, 2017
USD ($)
Operating leases            
Area of office and laboratory space leased | ft²   21,488        
Term of option to extend the lease         5 years  
Rent expense     $ 0.2 $ 0.2 $ 0.7 $ 0.7
Letter of credit outstanding   $ 0.5        
Refund received from landlord of security deposit on lease $ 0.2          
Letter of credit outstanding classified as restricted cash     $ 0.3   $ 0.3