Summary of Significant Accounting Policies - Additional Information (Detail) - USD ($) $ in Thousands	6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jan. 01, 2018	Dec. 31, 2017
Summary Of Significant Accounting Policies [Line Items]
Cumulative adjustment to accumulated deficit				$ 207
Decrease in cash flows from financing activities as compared to previously reported amounts	$ (111,261)	$ (10,425)
Accumulated Deficit
Summary Of Significant Accounting Policies [Line Items]
Cumulative adjustment to accumulated deficit				$ 207
ASU 2014-09 | ISU Abxis | Difference between Revenue Guidance in Effect before and after Topic 606 | Accumulated Deficit
Summary Of Significant Accounting Policies [Line Items]
Cumulative adjustment to accumulated deficit			$ 200
ASU 2016-18
Summary Of Significant Accounting Policies [Line Items]
Decrease in cash flows from financing activities as compared to previously reported amounts		$ 13,600