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The Company - Additional Information (Detail) - USD ($) $ / shares in Units, $ in Thousands			12 Months Ended
	Aug. 24, 2017	Apr. 11, 2017	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Business Description [Line Items]
Net proceeds from offering			$ 29,847	$ 34,910	$ 9,881
Follow On Offering
Business Description [Line Items]
Common stock shares sold	9,200,000	5,228,750
Public offering price	$ 40.75	$ 22.95
Net proceeds from offering	$ 356,200	$ 115,100
Underwriting discounts and commissions	18,700	4,900
Offering expenses	$ 400	$ 600

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