Summary of Significant Accounting Policies - Schedule of Cumulative Effect of Changes to Condensed Consolidated Balance Sheet for Adoption of ASC 606 (Detail) - USD ($) $ in Thousands	Jun. 30, 2018	Jan. 01, 2018	Dec. 31, 2017
Assets
Accounts receivable	$ 40,939		$ 19,967
Prepaid expenses and other current assets	5,641		9,118
Other assets	3,142		1,760
Liabilities:
Deferred revenue	10,720		2,212
Roche related-party deferred revenue	8,190		3,742
Stockholders’ equity:
Accumulated deficit	(558,851)		$ (506,349)
ASU 2014-09 [Member]
Assets
Accounts receivable		$ 37,210
Prepaid expenses and other current assets		9,828
Other assets		2,333
Liabilities:
Deferred revenue		2,368
Roche related-party deferred revenue		3,820
Stockholders’ equity:
Accumulated deficit		(488,057)
ASU 2014-09 [Member] | Adjustments Due to ASC 606 [Member]
Assets
Accounts receivable	(12,077)	17,243
Prepaid expenses and other current assets	(177)	710
Other assets	(357)	573
Liabilities:
Deferred revenue	(136)	156
Roche related-party deferred revenue	(1,259)	78
Stockholders’ equity:
Accumulated deficit	$ (11,216)	$ 18,292