Balance Sheet Components - Property and Equipment, Net (Details) - USD ($) $ in Thousands	12 Months Ended
Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Property, Plant and Equipment [Line Items]
Property and equipment, gross	$ 59,005	$ 37,172
Less: accumulated depreciation	(12,273)	(12,010)
Total property and equipment, net	46,732	25,162
Depreciation expense	7,991	7,415	$ 3,082
Lab equipment
Property, Plant and Equipment [Line Items]
Property and equipment, gross	21,846	14,352
Leasehold improvements
Property, Plant and Equipment [Line Items]
Property and equipment, gross	34,563	22,288
Computers equipment and purchased software
Property, Plant and Equipment [Line Items]
Property and equipment, gross	1,156	458
Furniture and fixtures
Property, Plant and Equipment [Line Items]
Property and equipment, gross	$ 1,440	$ 74

Amount of accumulated depreciation, depletion and amortization for physical assets used in the normal conduct of business to produce goods and services.

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The aggregate expense recognized in the current period that allocates the cost of tangible assets, intangible assets, or depleting assets to periods that benefit from use of the assets.

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Amount before accumulated depreciation, depletion and amortization of physical assets used in the normal conduct of business and not intended for resale. Examples include, but are not limited to, land, buildings, machinery and equipment, office equipment, and furniture and fixtures.

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