Collaboration Agreements - Additional Information (Detail) $ / shares in Units, $ in Thousands, shares in Millions	1 Months Ended		3 Months Ended	7 Months Ended	12 Months Ended				34 Months Ended
	Aug. 31, 2017 USD ($) employee $ / shares shares	Mar. 31, 2014 USD ($)	Dec. 31, 2017 USD ($)	Dec. 31, 2015 USD ($)	Dec. 31, 2017 USD ($) $ / shares	Dec. 31, 2016 USD ($)	Dec. 31, 2015 USD ($)	Dec. 31, 2014 USD ($)	Dec. 31, 2016 USD ($)
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Amount of certain milestones achieved in account receivable			$ 19,564		$ 19,564	$ 22,193			$ 22,193
Revenue recognized under collaboration agreement					42,895	17,355	$ 6,028
Deferred revenue, net of current portion			3,304		3,304	22,664			22,664
Estimated deferred revenue recognized within one year			9,229		$ 9,229	19,033			19,033
Exercise Price Of Common Stock Warrants | $ / shares					$ 10.96
Celgene Corporation | Collaborative Arrangement
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Milestone payments		$ 21,500
Revenue recognized under collaboration agreement							2,200
Deferred revenue recorded under collaboration agreement			5,400		$ 5,400
License and Services Revenue					200	3,200
Estimated deferred revenue recognized within one year			3,700		$ 3,700
Collaborative agreement period					1 year
Celgene Corporation | Collaborative Arrangement | Upfront Payment Arrangement
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Collaboration agreement upfront payment		5,800							$ 2,100
Proceeds from Collaborators								$ 6,000
Celgene Corporation | Collaborative Arrangement | Maximum
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Milestone payments		$ 27,300
Merck Sharp & Dohme Corp. | Collaborative Arrangement
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Proceeds from Collaborators					$ 6,800	8,700
Deferred revenue recorded under collaboration agreement			1,100		$ 1,100
Collaborative agreement period					1 year
Merck Sharp & Dohme Corp. | Collaborative Arrangement | Upfront Payment Arrangement
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Proceeds from Collaborators				$ 3,900		12,000
License and Services Revenue						8,500
Merck Sharp & Dohme Corp. | Collaborative Arrangement | Development Funding
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Proceeds from Collaborators							3,900
Merck Sharp & Dohme Corp. | Clinical Collaborative Arrangement
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
License and Services Revenue					$ 27,000	8,600	$ 3,700
Estimated deferred revenue recognized within one year			1,000		1,000
Medivation, Inc. and Astellas Pharma, Inc. | Collaborative Arrangement
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Proceeds from Collaborators					900	2,400
License and Services Revenue					11,500	4,800
Termination penalty					1,000
Medivation, Inc. and Astellas Pharma, Inc. | Collaborative Arrangement | Upfront Payment Arrangement
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Proceeds from Collaborators						$ 6,000
Lam Research Corporation [Member] | Collaborative Arrangement
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Proceeds from Collaborators			13,400
License and Services Revenue					3,700
Maximum Amount Of Royalties Payable, Ratio To License And Service Revenue	3
Reimbursement Of Counterparty Costs, Maximum Number Of Employees | employee	10
Exercise Price Of Common Stock Warrants | $ / shares	$ 16.75
Stock and Warrants Issued During Period, Value, Preferred Stock and Warrants	$ 6,700
Other Liabilities, Current			$ 8,300		$ 8,300
Lam Research Corporation [Member] | Collaborative Arrangement | Maximum
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
License and Services Revenue	$ 50,000
Class of Warrant or Right, Outstanding | shares	1.0