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Summary of significant accounting policies.

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Summary of Significant Accounting Policies - Additional Information (Detail) - USD ($)	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Jan. 01, 2019	Jan. 01, 2018
Summary Of Significant Accounting Policies [Line Items]
Increase in cash flows from financing activities as compared to previously reported amounts	$ 111,332,000	$ 21,082,000
Cumulative adjustment to accumulated deficit		207,000
Lease practical expedients	true
Derivative liability		0
Reduction to research and development expenses	$ 50,000	100,000
Software
Summary Of Significant Accounting Policies [Line Items]
Estimated useful lives	3 years
Computer equipment
Summary Of Significant Accounting Policies [Line Items]
Estimated useful lives	3 years
Furniture \| Minimum
Summary Of Significant Accounting Policies [Line Items]
Estimated useful lives	3 years
Furniture \| Maximum
Summary Of Significant Accounting Policies [Line Items]
Estimated useful lives	7 years
Leasehold improvements \| Minimum
Summary Of Significant Accounting Policies [Line Items]
Estimated useful lives	3 years
Leasehold improvements \| Maximum
Summary Of Significant Accounting Policies [Line Items]
Estimated useful lives	7 years
Subsequent Event
Summary Of Significant Accounting Policies [Line Items]
Right of use assets			$ 2,500,000
Lease liabilities			$ 2,300,000
Accumulated Deficit
Summary Of Significant Accounting Policies [Line Items]
Cumulative adjustment to accumulated deficit		$ 207,000
ISU Abxis
Summary Of Significant Accounting Policies [Line Items]
Accounts receivable	$ 10,000
ASU 2016-18
Summary Of Significant Accounting Policies [Line Items]
Increase in cash flows from financing activities as compared to previously reported amounts	$ 5,300,000
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				$ 200,000