Revenue - Revenue Disaggregated by Type of Service (Detail) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
Molecular Information Services [Member]
Disaggregation Of Revenue [Line Items]
Revenue	$ 38,702	$ 24,777	$ 70,478	$ 40,371
Pharma Research and Development Services [Member]
Disaggregation Of Revenue [Line Items]
Revenue	3,732	1,215	8,514	2,302
Third Party [Member]
Disaggregation Of Revenue [Line Items]
Revenue	42,434	25,992	78,992	42,673
Third Party [Member] | Molecular Information Services [Member]
Disaggregation Of Revenue [Line Items]
Revenue	38,702	24,777	70,478	40,371
Third Party [Member] | Molecular Information Services [Member] | Clinical Sample Profiling Services [Member]
Disaggregation Of Revenue [Line Items]
Revenue	20,709	11,977	36,298	22,626
Third Party [Member] | Molecular Information Services [Member] | Pharma Sample Profiling Services [Member]
Disaggregation Of Revenue [Line Items]
Revenue	15,746	10,697	28,852	13,151
Third Party [Member] | Molecular Information Services [Member] | Other Molecular Information Services [Member]
Disaggregation Of Revenue [Line Items]
Revenue	2,247	2,103	5,328	4,594
Third Party [Member] | Pharma Research and Development Services [Member]
Disaggregation Of Revenue [Line Items]
Revenue	3,732	1,215	8,514	2,302
Third Party [Member] | Pharma Research and Development Services [Member] | R&D and Regulatory Services [Member]
Disaggregation Of Revenue [Line Items]
Revenue	3,732	1,215	8,514	2,302
Roche [Member]
Disaggregation Of Revenue [Line Items]
Related-party revenue	14,572	9,012	30,854	18,659
Roche [Member] | Molecular Information Services [Member]
Disaggregation Of Revenue [Line Items]
Related-party revenue	12,005	5,520	26,820	11,024
Roche [Member] | Molecular Information Services [Member] | Clinical Sample Profiling Services [Member]
Disaggregation Of Revenue [Line Items]
Related-party revenue	3,089	964	6,287	1,934
Roche [Member] | Molecular Information Services [Member] | Pharma Sample Profiling Services [Member]
Disaggregation Of Revenue [Line Items]
Related-party revenue	6,277	3,556	16,727	7,090
Roche [Member] | Molecular Information Services [Member] | Other Molecular Information Services [Member]
Disaggregation Of Revenue [Line Items]
Related-party revenue	2,639	1,000	3,806	2,000
Roche [Member] | Pharma Research and Development Services [Member]
Disaggregation Of Revenue [Line Items]
Related-party revenue	2,567	3,492	4,034	7,635
Roche [Member] | Pharma Research and Development Services [Member] | R&D and Regulatory Services [Member]
Disaggregation Of Revenue [Line Items]
Related-party revenue	$ 2,567	$ 3,492	$ 4,034	$ 7,635