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Significant Agreements - Additional Information (Detail)				1 Months Ended			3 Months Ended		6 Months Ended
	Apr. 06, 2016	Apr. 07, 2015 Director Designee	Jan. 11, 2015 USD ($) $ / shares shares	Jun. 30, 2017 Director	Feb. 28, 2017 Director	Mar. 31, 2014	Jun. 30, 2018 USD ($) shares	Jun. 30, 2017 USD ($)	Jun. 30, 2018 USD ($) shares	Jun. 30, 2017 USD ($)	Dec. 31, 2017 shares
Open Option Contracts Written [Line Items]
Common stock, shares outstanding \| shares							37,147,273		37,147,273		36,541,770
Number of directors \| Director		9		9	10
Revenue							$ 57,006,000	$ 35,004,000	$ 109,846,000	$ 61,332,000
Roche [Member]
Open Option Contracts Written [Line Items]
Primary investment in cash			$ 250,000,000
Common stock shares sold \| shares			5,000,000
Common stock purchase price \| $ / shares			$ 50.00
Common stock, shares outstanding \| shares			15,604,288
Ownership percentage on common stock		61.30%					56.60%		56.60%
Number of designees \| Designee		3
R&D collaboration agreement period			5 years
Roche [Member] \| Molecular Information Platform Program [Member]
Open Option Contracts Written [Line Items]
Allocable consideration related to Molecular Information Platform			$ 85,000,000
Roche [Member] \| Circulating Tumor DNA Platform Program [Member]
Open Option Contracts Written [Line Items]
Clinical milestone payments recognized									$ 12,000,000
Roche [Member] \| Companion Diagnostics (CDx) Development Program [Member]
Open Option Contracts Written [Line Items]
Clinical milestone payments recognized									600,000
Roche [Member] \| Ex-U.S. Commercialization Agreement [Member]
Open Option Contracts Written [Line Items]
Clinical milestone payments recognized									10,000,000
Roche [Member] \| Ex-U.S. Commercialization Agreement [Member] \| Service [Member]
Open Option Contracts Written [Line Items]
Aggregate gross margin on commercial sales									100,000,000
Roche [Member] \| IVD Collaboration Agreement [Member]
Open Option Contracts Written [Line Items]
Collaboration agreement expiration date	Apr. 07, 2020
Collaboration agreement additional extension term	2 years
Roche [Member] \| Minimum [Member]
Open Option Contracts Written [Line Items]
R&D collaboration agreement amount			$ 150,000,000
Roche [Member] \| Minimum [Member] \| IVD Collaboration Agreement [Member]
Open Option Contracts Written [Line Items]
Ownership percentage on common stock	50.10%
Roche [Member] \| Maximum [Member] \| Immunotherapy Testing Platform Development Program [Member]
Open Option Contracts Written [Line Items]
Clinical milestone payments recognized									6,600,000
Biopharmaceutical Partner [Member] \| Master Services Agreement [Member]
Open Option Contracts Written [Line Items]
Amendment period						5 years
Biopharmaceutical Partner [Member] \| Master Services Agreement [Member] \| Significant Long Term Agreements [Member] \| Sample Profiling and Milestone Payments [Member]
Open Option Contracts Written [Line Items]
Revenue							$ 3,059,000	$ 830,000	$ 5,384,000	$ 1,300,000