Summary of Significant Accounting Policies - Adoption of ACS 606 (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2019	Jun. 30, 2018	Jun. 30, 2019	Jun. 30, 2018	Jan. 01, 2019	Dec. 31, 2018
Revenue, Initial Application Period Cumulative Effect Transition [Line Items]
Accounts receivable	$ 40,363		$ 40,363		$ 40,597	$ 35,690
Accumulated deficit	(314,809)		(314,809)		(275,892)	$ (280,799)
Precision oncology testing	42,064	$ 17,822	70,901	$ 32,013
Development services	11,911	1,560	19,729	4,061
Total revenue	53,975	$ 19,382	90,630	$ 36,074
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	655		655
Accumulated deficit	655		655
Precision oncology testing	1,023		2,017
Development services	(1,362)		(1,362)
Total revenue	(339)		655
Balances Without Adoption of ASC606
Revenue, Initial Application Period Cumulative Effect Transition [Line Items]
Accounts receivable	41,018		41,018
Accumulated deficit	(314,154)		(314,154)
Precision oncology testing	43,087		72,918
Development services	10,549		18,367
Total revenue	$ 53,636		$ 91,285