Summary of Significant Accounting Policies - Adoption of ACS 606 (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2019	Sep. 30, 2018	Jan. 01, 2019	Dec. 31, 2018
Revenue, Initial Application Period Cumulative Effect Transition [Line Items]
Accounts receivable	$ 39,689		$ 39,689		$ 40,597	$ 35,690
Accumulated deficit	(327,599)		(327,599)		(275,892)	$ (280,799)
Precision oncology testing	52,147	$ 18,298	123,048	$ 50,311
Development services	8,701	3,394	28,430	7,455
Total revenue	60,848	$ 21,692	151,478	$ 57,766
Topic 606 adoption
Revenue, Initial Application Period Cumulative Effect Transition [Line Items]
Accounts receivable					4,907
Accumulated deficit					$ 4,907
Effect of Change | Topic 606 adoption
Revenue, Initial Application Period Cumulative Effect Transition [Line Items]
Accounts receivable	(501)		(501)
Accumulated deficit	(501)		(501)
Precision oncology testing	(439)		1,578
Development services	(717)		(2,079)
Total revenue	(1,156)		(501)
Balances Without Adoption of ASC606
Revenue, Initial Application Period Cumulative Effect Transition [Line Items]
Accounts receivable	39,188		39,188
Accumulated deficit	(328,100)		(328,100)
Precision oncology testing	51,708		124,626
Development services	7,984		26,351
Total revenue	$ 59,692		$ 150,977