Schedule of Cash as Reported in Condensed Consolidated Statements of Cash Flows (Detail) - USD ($) $ in Thousands	Mar. 31, 2018	Dec. 31, 2017	Mar. 31, 2017	Dec. 31, 2016
Accounting Policies [Abstract]
Cash and cash equivalents	$ 168,414	$ 181,568	$ 135,669	$ 188,480
Restricted cash (SRX Cardio)	47	173	174	178
Total cash balance in condensed consolidated statements of cash flows	$ 168,461	$ 181,741	$ 135,843	$ 188,658