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	Sep. 30, 2013 $ / shares shares	Oct. 31, 2016 Director	Aug. 31, 2016	Mar. 31, 2014 USD ($) Deliverable	May 31, 2012 USD ($)	Dec. 31, 2016 USD ($) shares	Sep. 30, 2016 USD ($)	Jun. 30, 2016 USD ($)	Mar. 31, 2016 USD ($)	Dec. 31, 2015 USD ($) shares	Sep. 30, 2015 USD ($)	Jun. 30, 2015 USD ($)	Mar. 31, 2015 USD ($)	Dec. 31, 2016 USD ($) Deliverable Unit shares	Dec. 31, 2015 USD ($) shares	Dec. 31, 2014 USD ($)	Oct. 31, 2013 USD ($)
Open Option Contracts Written [Line Items]
Common stock shares sold | shares				6,772,221
Common stock purchase price | $ / shares				$ 18.00
Common stock, shares outstanding | shares									35,281,001				34,513,845				35,281,001	34,513,845
Number of directors | Director		9
Number of existing independent directors | Director		4
Number of new independent director | Director					1
Aggregate gross margin on commercial sales																	$ 73,388,000	$ 79,759,000	$ 60,221,000
Amendment period						2 years		2 years
Amendment fee								$ 14,200,000
Second amendment extension term																	2016-09
Second amendment fee																				$ 17,100,000
Number of deliverables | Deliverable																	3
Number of accounting units | Unit																	1
Revenue recognition period																	3 years
Revenue									$ 28,824,000	$ 29,426,000	$ 28,237,000	$ 30,378,000	$ 26,051,000	$ 25,399,000	$ 22,458,000	$ 19,295,000	$ 116,865,000	93,203,000	61,079,000
Deferred revenue									$ 2,104,000				$ 717,000				2,104,000	717,000		$ 0
Significant Long Term Agreements [Member]
Open Option Contracts Written [Line Items]
Revenue																	3,719,000	6,968,000	7,996,000
Significant Long Term Agreements [Member] | Sample Profiling and Milestone Payments [Member]
Open Option Contracts Written [Line Items]
Revenue																	$ 11,735,000	$ 10,618,000
Significant Long Term Agreements [Member] | Sample Profiling [Member]
Open Option Contracts Written [Line Items]
Revenue																			$ 2,689,000
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%							59.60%								59.60%
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
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
Number of deliverables | Deliverable							7
Deferred revenue							$ 0