Background and Organization - Additional Information (Detail) - USD ($) $ in Thousands	3 Months Ended	9 Months Ended
Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017	Dec. 31, 2017
Organization Consolidation And Presentation Of Financial Statements [Abstract]
Net loss	$ (32,852)	$ (11,216)	$ (59,656)	$ (47,530)
Net cash used from operations			(77,429)	$ (43,694)
Cash and cash equivalents	$ 201,543		$ 201,543		$ 57,389

The portion of profit or loss for the period, net of income taxes, which is attributable to the parent.

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