Schedule of Cash as Reported in Condensed Consolidated Statements of Cash Flows (Detail) - USD ($)	Sep. 30, 2018	Dec. 31, 2017	Sep. 30, 2017	Dec. 31, 2016
Restricted Cash And Cash Equivalents Items [Line Items]
Cash and cash equivalents	$ 146,700,000	$ 181,568,000	$ 344,021,000	$ 188,480,000
Restricted cash	272,000	173,000
Total cash balance in condensed consolidated statements of cash flows	146,972,000	181,741,000	344,191,000	188,658,000
SRX Cardio
Restricted Cash And Cash Equivalents Items [Line Items]
Restricted cash	30,000	$ 173,000	$ 170,000	$ 178,000
Royalty Payments | HealthCare Royalty Partners and its Affiliates (“HCR”)
Restricted Cash And Cash Equivalents Items [Line Items]
Restricted cash	$ 242,000