Income Taxes - Additional Information (Detail) - USD ($) $ in Thousands	3 Months Ended	6 Months Ended	12 Months Ended
Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017	Dec. 31, 2017
Income Tax Disclosure [Abstract]
Pre-tax losses	$ (17,346)	$ (9,302)	$ (34,159)	$ (31,537)
Benefit from income taxes	$ 152	$ 2	$ 198	$ 33
Percentage of annual research and development credit for cash			65.00%
Income tax rate			21.00%		35.00%

Amount of income (loss) from continuing operations, including income (loss) from equity method investments, before deduction of income tax expense (benefit), and income (loss) attributable to noncontrolling interest.

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