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Collaboration and Other Agreements - Roche (Details) $ in Thousands	1 Months Ended		3 Months Ended		6 Months Ended
	Jan. 31, 2018 USD ($)	Dec. 31, 2017 USD ($) performance_obligation	Jun. 30, 2019 USD ($)	Jun. 30, 2018 USD ($)	Jun. 30, 2019 USD ($)	Jun. 30, 2018 USD ($)	Dec. 31, 2018 USD ($)
Collaboration And Other Agreements [Line Items]
Revenues			$ 10,593	$ 18,834	$ 20,255	$ 23,529
Deferred revenue, current			14,449		14,449		$ 21,721
Deferred revenue, net of current portion			15,413		15,413		19,001
F. Hoffmann-La Roche Ltd. and Hoffmann-La Roche Inc.
Collaboration And Other Agreements [Line Items]
Non-refundable upfront payment	$ 10,000
Potential milestone payments and royalties on future sales under agreement		$ 370,000
Number of performance obligations | performance_obligation		1
Collaborative agreement transaction price		$ 10,000
Upfront payment recognition period		30 months
Deferred revenue							6,000
Deferred revenue, current			4,000		$ 4,000		4,000
Deferred revenue, net of current portion							$ 2,000
Revenues From License Agreements | F. Hoffmann-La Roche Ltd. and Hoffmann-La Roche Inc.
Collaboration And Other Agreements [Line Items]
Revenues			$ 1,000	$ 1,000

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