Basis of Presentation, Principles of Consolidation and Summary of Significant Accounting Policies (Details) - USD ($) $ in Thousands	Sep. 30, 2019	Jun. 30, 2019	Dec. 31, 2018	Sep. 30, 2018	Dec. 31, 2017
Cash, Cash Equivalents and Restricted Cash [Abstract]
Cash and cash equivalents	$ 89,195		$ 111,355
Restricted cash	1,525		1,436
Total	90,720		112,791	$ 39,721	$ 18,349
Leases [Abstract]
Operating lease, right-of-use assets	2,254		$ 0
Operating lease, liability	2,743
Government Grants [Abstract]
Receivable of offset grants against expenses	400	$ 1,100
ASU 2016-02 [Member]
Leases [Abstract]
Operating lease, right-of-use assets	2,600
Operating lease, liability	3,100
Reclassification of deferred rent to right-of-use assets	$ 500