Basis of Presentation, Principles of Consolidation and Summary of Significant Accounting Policies (Details) - USD ($) $ in Thousands	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2018	Jun. 30, 2018	Dec. 31, 2017
Cash, Cash Equivalents and Restricted Cash [Abstract]
Cash and cash equivalents	$ 64,833		$ 111,355
Restricted cash	1,525		1,436
Total	66,358		112,791	$ 54,192	$ 18,349
Leases [Abstract]
Operating lease, right-of-use assets	2,452		$ 0
Operating lease, liability	2,846
Government Grants [Abstract]
Receivable of offset grants against expenses	800	$ 800
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