Balance Sheet Account Details (Property and Equipment) (Details) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Property, Plant and Equipment [Line Items]
Total property and equipment, gross	$ 26,803	$ 4,229
Accumulated depreciation and amortization	(2,235)	(1,084)
Total property and equipment, net	24,568	3,145
Lab equipment
Property, Plant and Equipment [Line Items]
Total property and equipment, gross	7,812	2,610
Furniture and fixtures
Property, Plant and Equipment [Line Items]
Total property and equipment, gross	2,282	389
Computer hardware
Property, Plant and Equipment [Line Items]
Total property and equipment, gross	1,238	457
Software
Property, Plant and Equipment [Line Items]
Total property and equipment, gross	619	309
Leasehold improvements
Property, Plant and Equipment [Line Items]
Total property and equipment, gross	$ 14,852	$ 464