Intangible Assets - Additional Information (Detail) - USD ($)	3 Months Ended	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Goodwill And Intangible Assets [Line Items]
Indefinite-lived intangible assets		$ 0	$ 326,359,000
Proceeds from sale of intangible asset		25,000,000	50,000,000	$ 125,000,000
Medivation
Goodwill And Intangible Assets [Line Items]
Milestone Payments Received		$ 25,000,000.0	50,000,000.0
IPR&D
Goodwill And Intangible Assets [Line Items]
Finite-lived intangible assets useful life		9 years
In Process Research and Development of Palynziq | Europe
Goodwill And Intangible Assets [Line Items]
Indefinite-lived intangible assets			$ 326,400,000
IPR&D of Kyndrisa and Other Exon
Goodwill And Intangible Assets [Line Items]
Indefinite-lived intangible assets		$ 0
Rare Pediatric Disease Priority Review Voucher
Goodwill And Intangible Assets [Line Items]
Proceeds from sale of intangible asset				$ 125,000,000.0
IPR&D
Goodwill And Intangible Assets [Line Items]
Impairment charge	$ 5,800,000