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	Feb. 28, 2017 Director	Mar. 31, 2014 USD ($) Deliverable	Jun. 30, 2017 USD ($) shares	Jun. 30, 2016 USD ($)	Jun. 30, 2017 USD ($) shares	Jun. 30, 2016 USD ($)	Dec. 31, 2016 USD ($) shares
Open Option Contracts Written [Line Items]
Common stock, shares outstanding | shares						35,938,814		35,938,814		35,281,001
Number of directors | Director		9		10
Deferred revenue						$ 1,926,000		$ 1,926,000		$ 2,104,000
Revenue						35,004,000	$ 28,237,000	61,332,000	$ 58,615,000
Significant Long Term Agreements [Member] | Sample Profiling and Royalties Earned [Member]
Open Option Contracts Written [Line Items]
Revenue						$ 830,000		$ 1,300,000
Significant Long Term Agreements [Member] | Sample Profiling and Milestone Payments [Member]
Open Option Contracts Written [Line Items]
Revenue							$ 4,040,000		$ 6,864,000
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%				58.50%		58.50%
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] | 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
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]
Open Option Contracts Written [Line Items]
Clinical milestone payments recognized								$ 13,000,000
Roche [Member] | Maximum [Member] | Immunotherapy Testing Platform Development Program [Member]
Open Option Contracts Written [Line Items]
Clinical milestone payments recognized								6,600,000
Roche [Member] | Maximum [Member] | Circulating Tumor DNA Platform Program [Member]
Open Option Contracts Written [Line Items]
Clinical milestone payments recognized								$ 12,000,000
Biopharmaceutical Partner [Member] | Master Services Agreement [Member]
Open Option Contracts Written [Line Items]
Amendment period					5 years
Deferred revenue					$ 0
Number of deliverables | Deliverable					7