Schedule of Indefinite Lived Intangible Assets (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015
Indefinite-lived Intangible Assets [Line Items]
Net carrying value	$ 332,199	$ 607,548
Pegvaliase
Indefinite-lived Intangible Assets [Line Items]
In-process research and development acquired	326,359	0
Kyndrisa
Indefinite-lived Intangible Assets [Line Items]
In-process research and development acquired	0	533,064
Other exons acquired with Prosensa
Indefinite-lived Intangible Assets [Line Items]
In-process research and development acquired	0	41,044
Reveglucosidase Alfa
Indefinite-lived Intangible Assets [Line Items]
In-process research and development acquired	0	25,010
Other Acquired Pre-Clinical Compounds
Indefinite-lived Intangible Assets [Line Items]
In-process research and development acquired	$ 5,840	$ 8,430