Summary of Significant Accounting Policies - Schedule of Cash as Reported in Condensed Consolidated Statements of Cash Flows (Detail) - USD ($) $ in Thousands	Sep. 30, 2019	Dec. 31, 2018	Sep. 30, 2018	Dec. 31, 2017
Restricted Cash and Cash Equivalents Items [Line Items]
Cash and cash equivalents	$ 304,265	$ 138,951	$ 146,700	$ 181,568
Restricted cash	3,201	1,062
Total cash balance in condensed consolidated statements of cash flows	307,466	140,013	146,972	181,741
Security Deposit for Office Lease
Restricted Cash and Cash Equivalents Items [Line Items]
Restricted cash	46	0	0	0
Royalty Payments | HealthCare Royalty Partners and its Affiliates (“HCR”)
Restricted Cash and Cash Equivalents Items [Line Items]
Restricted cash	3,155	1,032	242	0
SRX Cardio
Restricted Cash and Cash Equivalents Items [Line Items]
Restricted cash	$ 0	$ 30	$ 30	$ 173