Notes Payable and Debt - Additional Information (Detail) - Roche Credit Facility [Member] - USD ($)			12 Months Ended
	Jul. 31, 2017	Aug. 02, 2016	Dec. 31, 2017	Dec. 31, 2016
Line Of Credit Facility [Line Items]
Credit facility, borrowing capacity	$ 200,000,000	$ 100,000,000
Credit facility, available borrowing capacity	$ 80,000,000
Credit facility, remaining borrowing capacity			$ 90,000,000
Credit facility, quarterly commitment fee percentage on available balance, payable during draw period	0.40%	0.30%
Credit facility, draw period	4 years	3 years
Credit facility, interest rate	6.50%	5.00%
Credit facility, frequency of periodic payment			quarterly
Credit facility, maturity date	Feb. 02, 2026
Credit facility, payments description			The Company shall pay Roche quarterly during the Draw Period and for six months thereafter accrued interest on the outstanding principal of the loans. Beginning six months after the Draw Period and for five years thereafter, the Company shall pay Roche quarterly equal payments of principal, with accrued interest, in arrears until maturity of the Roche Credit Facility on February 2, 2026 (the “Final Maturity Date”).
Credit facility, covenants description			The Roche Credit Facility contains certain affirmative covenants, including, among others, obligations for the Company to provide monthly and annual financial statements, to meet specified minimum cash requirements, to provide tax gross-up and indemnification protection, and to comply with laws. The Roche Credit Facility also contains certain negative covenants, including, among others, restrictions on the Company’s ability to dispose of certain assets, to acquire another company or business, to encumber or permit liens on certain assets, to incur additional indebtedness (subject to customary exceptions), and to pay dividends on the Company’s common stock. The Company was in compliance with its covenants under the Roche Credit Facility as of December 31, 2017.
Debt Default, Short-term Debt, Amount			$ 0
Credit facility, borrowings outstanding			60,000,000
Interest expense			1,300,000	$ 100,000
Loss on extinguishment of debt due to write-off unamortized deferred financing costs			$ 300,000
Foreign Subsidiaries [Member]
Line Of Credit Facility [Line Items]
Equity interest percentage held		65.00%
Achievement of Certain Milestones [Member]
Line Of Credit Facility [Line Items]
Credit facility, available borrowing capacity	$ 70,000,000
Achievement of Certain Additional Milestones [Member]
Line Of Credit Facility [Line Items]
Credit facility, remaining borrowing capacity	$ 50,000,000