Stockholders' Equity - Additional Information (Detail) - USD ($)	1 Months Ended		9 Months Ended
	Aug. 31, 2016	Jan. 31, 2016	Sep. 30, 2017	Sep. 30, 2016	Dec. 31, 2016
Class Of Stock [Line Items]
Net proceeds from sale of common stock	$ 215,900,000	$ 281,600,000	$ 26,397,000	$ 515,506,000
Share of common stock issued	6,969,696	10,344,827
Common stock, par value			$ 0.0001		$ 0.0001
Value of shares purchased by Baker Entities in public offerings			$ 12,000		$ 12,000
Option To Purchase Additional Shares
Class Of Stock [Line Items]
Share of common stock issued	909,090
Registration Rights Agreement
Class Of Stock [Line Items]
Common stock, par value		$ 0.0001
Expiration period		10 years
Maximum amount of expense agreed to bear under the agreement		$ 50,000
Registration Rights Agreement | Baker Brothers Life Sciences L.P.
Class Of Stock [Line Items]
Value of shares purchased by Baker Entities in public offerings	$ 43,000,000	$ 75,000,000