Product Revenues, Net - Summary of Net Product Revenues Of ONPATTRO (Detail) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2019	Sep. 30, 2018
Disaggregation of Revenue [Line Items]
Total revenues	$ 70,061	$ 2,069	$ 148,069	$ 53,875
Product Revenues, Net
Disaggregation of Revenue [Line Items]
Total revenues	46,066	460	110,588	460
Product Revenues, Net | United States
Disaggregation of Revenue [Line Items]
Total revenues	33,591	460	80,543	460
Product Revenues, Net | Rest of World
Disaggregation of Revenue [Line Items]
Total revenues	$ 12,475	$ 0	$ 30,045	$ 0