Deferred Revenue - Additional Information (Details) - USD ($) $ in Thousands		6 Months Ended
	Jun. 30, 2019	Jun. 30, 2019	Dec. 31, 2018
Disaggregation Of Revenue [Line Items]
Current deferred revenue	$ 61,194	$ 61,194	$ 12,695
Deferred revenue, less current portion	240,919	240,919	704
Revenue recognized	4,400	19,341
Development Revenue
Disaggregation Of Revenue [Line Items]
Current deferred revenue	46,578	46,578	1,457
Deferred revenue, less current portion	240,426	240,426	188
Development Revenue | Genentech, Inc.
Disaggregation Of Revenue [Line Items]
Current deferred revenue	45,000	45,000
Deferred revenue, less current portion	240,200	240,200
Sequencing Revenue
Disaggregation Of Revenue [Line Items]
Current deferred revenue	14,616	14,616	11,238
Deferred revenue, less current portion	$ 493	493	$ 516
Revenue recognized		$ 800