Immaterial Restatement Of Prior Period Financial Statements For Stock-Based Compensation - Summary of Revisions to the Consolidated Statements of Operations (Detail) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Operating costs and expenses:
Selling, general and administrative	$ 19,837	$ 18,527	$ 67,393	$ 55,052
Research and development	21,060	13,815	60,442	43,760
Total operating costs and expenses	54,292	51,449	168,530	155,369
Loss from operations	(29,024)	(15,054)	(88,600)	(55,572)
Loss before income taxes	(68,716)	(19,759)	(70,784)	(64,103)
Net loss	$ (68,718)	$ (18,293)	$ (70,792)	$ (62,691)
Net loss per share:
Basic ($ per share)	$ (0.66)	$ (0.22)	$ (0.69)	$ (0.78)
Diluted ($ per share)	$ (0.66)	$ (0.22)	$ (0.69)	$ (0.78)
Previously Reported [Member]
Operating costs and expenses:
Selling, general and administrative		$ 18,880		$ 54,595
Research and development		13,878		43,670
Total operating costs and expenses		51,865		154,822
Loss from operations		(15,470)		(55,025)
Loss before income taxes		(20,175)		(63,556)
Net loss		$ (18,709)		$ (62,144)
Net loss per share:
Basic ($ per share)		$ (0.22)		$ (0.78)
Diluted ($ per share)		$ (0.22)		$ (0.78)