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Collaboration and Other Agreements - Roche (Details) $ in Thousands	1 Months Ended		3 Months Ended		9 Months Ended
	Jan. 31, 2018 USD ($)	Dec. 31, 2017 USD ($) performance_obligation	Sep. 30, 2018 USD ($)	Sep. 30, 2017 USD ($)	Sep. 30, 2018 USD ($)	Sep. 30, 2017 USD ($)	Jan. 01, 2018 USD ($)
Collaboration And Other Agreements [Line Items]
Revenues			$ 20,798	$ 1,663	$ 44,327	$ 5,383
Deferred revenue, current		$ 7,202	7,377		7,377		$ 7,742
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			7,000		7,000
Deferred revenue, current			4,000		4,000
Revenues From License Agreements \| F. Hoffmann-La Roche Ltd. and Hoffmann-La Roche Inc.
Collaboration And Other Agreements [Line Items]
Revenues			$ 1,000		$ 3,000