Intangible Assets and Goodwill - Summary of Definite-Lived Intangible Assets Subject to Amortization and Indefinite-Lived Intangible Assets (Detail) - USD ($) $ in Thousands	6 Months Ended	12 Months Ended
	Jun. 30, 2021	Dec. 31, 2020
Finite Lived Intangible Assets [Line Items]
Definite-lived intangible assets, accumulated amortization	$ (13,776)	$ (9,156)
Indefinite-lived intangible assets, gross carrying amount/net carrying amount	42,291	42,291
Total intangible assets, gross carrying amount	88,283	88,283
Total intangible assets, net carrying amount	$ 74,507	$ 79,127
In-Process Research and Development [Member] | MRT Product [Member] | Shire's MRT Program [Member]
Finite Lived Intangible Assets [Line Items]
Definite-lived intangible assets, estimated life	6 years	6 years
Definite-lived intangible assets, gross carrying amount	$ 45,992	$ 45,992
Definite-lived intangible assets, accumulated amortization	(13,776)	(9,156)
Definite-lived intangible assets, net carrying amount	$ 32,216	$ 36,836