Summary of Significant Accounting Policies - Schedule of Cash as Reported in Condensed Consolidated Statements of Cash Flows (Detail) - USD ($) $ in Thousands	Jun. 30, 2019	Dec. 31, 2018	Jun. 30, 2018	Dec. 31, 2017
Restricted Cash And Cash Equivalents Items [Line Items]
Cash and cash equivalents	$ 185,385	$ 138,951	$ 173,052	$ 181,568
Restricted cash	2,280	1,062
Total cash balance in condensed consolidated statements of cash flows	187,665	140,013	173,082	181,741
SRX Cardio
Restricted Cash And Cash Equivalents Items [Line Items]
Restricted cash		30	$ 30	$ 173
Royalty Payments | HealthCare Royalty Partners and its Affiliates (“HCR”)
Restricted Cash And Cash Equivalents Items [Line Items]
Restricted cash	$ 2,280	$ 1,032