Schedule of Cash as Reported in Condensed Consolidated Statements of Cash Flows (Detail) - USD ($) $ in Thousands	Jun. 30, 2018	Dec. 31, 2017	Jun. 30, 2017	Dec. 31, 2016
Accounting Policies [Abstract]
Cash and cash equivalents	$ 173,052	$ 181,568	$ 123,837	$ 188,480
Restricted cash (SRX Cardio)	30	173	172	178
Total cash balance in condensed consolidated statements of cash flows	$ 173,082	$ 181,741	$ 124,009	$ 188,658