Collaboration Revenue - Adjustments to Accounts on Condensed Consolidated Balance Sheet (Detail) - USD ($) $ in Thousands	Mar. 31, 2018	Jan. 01, 2018	Dec. 31, 2017
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Deferred revenue - related party	$ 17,859		$ 12,079
Deferred revenue, net of current portion - related party	102,333		84,847
Accumulated deficit	$ (318,347)		$ (263,571)
ASU 2014-09 [Member]
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Deferred revenue - related party		$ 17,418
Deferred revenue, net of current portion - related party		106,365
Accumulated deficit		(290,428)
Difference between Revenue Guidance in Effect before and after Topic 606 [Member] | ASU 2014-09 [Member]
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Deferred revenue - related party		5,339
Deferred revenue, net of current portion - related party		21,518
Accumulated deficit		$ (26,857)