Significant Agreements - Additional Information (Detail)				1 Months Ended		3 Months Ended								12 Months Ended
	Apr. 06, 2016	Apr. 07, 2015 USD ($) Director Designee $ / shares shares	Jan. 11, 2015 USD ($) $ / shares shares	Feb. 28, 2017 Director	Mar. 31, 2014 USD ($) Deliverable	Dec. 31, 2017 USD ($) shares	Sep. 30, 2017 USD ($)	Jun. 30, 2017 USD ($)	Mar. 31, 2017 USD ($)	Dec. 31, 2016 USD ($) shares	Sep. 30, 2016 USD ($)	Jun. 30, 2016 USD ($)	Mar. 31, 2016 USD ($)	Dec. 31, 2017 USD ($) shares	Dec. 31, 2016 USD ($) shares	Dec. 31, 2015 USD ($)
Open Option Contracts Written [Line Items]
Common stock, shares outstanding | shares						36,541,770				35,281,001				36,541,770	35,281,001
Number of directors | Director		9		10
Deferred revenue						$ 2,212,000				$ 2,104,000				$ 2,212,000	$ 2,104,000
Revenue						$ 48,913,000	$ 42,658,000	$ 35,004,000	$ 26,328,000	$ 28,824,000	$ 29,426,000	$ 28,237,000	$ 30,378,000	$ 152,903,000	116,865,000	$ 93,203,000
Roche [Member]
Open Option Contracts Written [Line Items]
Primary investment in cash		$ 250,000,000	$ 250,000,000
Common stock shares sold | shares		5,000,000	5,000,000
Common stock purchase price | $ / shares		$ 50.00	$ 50.00
Common stock, shares outstanding | shares		15,604,288	15,604,288
Ownership percentage on common stock		61.30%				57.50%								57.50%
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
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														$ 6,435,000	$ 11,735,000	$ 10,618,000