Commitments and Contingencies - Additional Information (Details) - USD ($) $ in Millions	1 Months Ended			3 Months Ended		9 Months Ended
	Feb. 28, 2018	Dec. 31, 2014	Jan. 31, 2010	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Commitments And Contingencies [Line Items]
Rent expense				$ 0.1	$ 0.1	$ 0.3	$ 0.3
Malvern, Pennsylvania
Commitments And Contingencies [Line Items]
Lease commencement date			Mar. 31, 2010
Lease expiration date			Jul. 31, 2015
Lease extended expiration date	Sep. 30, 2023	Jul. 31, 2018
Lessee, operating lease, option to extend description						In December 2014, the Malvern Lease was amended to extend its term to July 31, 2018. In February 2018, the Malvern lease was further amended to extend its term to September 30, 2023.
Lessee, operating lease, existence of option to extend [true false]						true