Fair Value of Financial Assets and Liabilities - Schedule of Total Acquisition Related Contingent Consideration Liability (Detail) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2021	Jun. 30, 2020	Jun. 30, 2021	Jun. 30, 2020
Business Acquisition, Contingent Consideration [Line Items]
Beginning Balance			$ 152,230
Discontinuation of MRT5201 and Decrease in fair value of contingent consideration	$ 4,242	$ 15,347	(39,737)	$ 5,895
Ending Balance	$ 112,493		112,493
Contingent Consideration
Business Acquisition, Contingent Consideration [Line Items]
Discontinuation of MRT5201 and Decrease in fair value of contingent consideration			$ (39,737)