Balance Sheet Accounts and Supplemental Disclosures - Schedule of Property and Equipment, Net (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2019	Dec. 31, 2018
Property Plant And Equipment [Line Items]
Total property and equipment	$ 4,576	$ 3,490
Less: accumulated depreciation	466	297
Property and equipment, net	4,110	3,193
Office Equipment
Property Plant And Equipment [Line Items]
Total property and equipment	$ 664	918
Office Equipment | Minimum
Property Plant And Equipment [Line Items]
Property and equipment, estimated useful life	3 years
Office Equipment | Maximum
Property Plant And Equipment [Line Items]
Property and equipment, estimated useful life	7 years
Computer Equipment
Property Plant And Equipment [Line Items]
Total property and equipment	$ 33	15
Property and equipment, estimated useful life	5 years
Software
Property Plant And Equipment [Line Items]
Total property and equipment	$ 50	50
Property and equipment, estimated useful life	3 years
Lab Equipment
Property Plant And Equipment [Line Items]
Total property and equipment	$ 1,870	1,070
Lab Equipment | Minimum
Property Plant And Equipment [Line Items]
Property and equipment, estimated useful life	2 years
Lab Equipment | Maximum
Property Plant And Equipment [Line Items]
Property and equipment, estimated useful life	5 years
Leasehold Improvements
Property Plant And Equipment [Line Items]
Total property and equipment	$ 1,314	1,243
Leasehold Improvements | Minimum
Property Plant And Equipment [Line Items]
Property and equipment, estimated useful life	6 years
Leasehold Improvements | Maximum
Property Plant And Equipment [Line Items]
Property and equipment, estimated useful life	7 years
Construction in Process
Property Plant And Equipment [Line Items]
Total property and equipment	$ 645	$ 194