Net Loss per Share - Computation of Denominators Used in Net Loss per Share (Detail) - shares	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Basic:
Weighted average shares outstanding - Basic	31,202,842	27,279,008	24,620,372
Diluted:
Weighted average shares outstanding - Basic	31,202,842	27,279,008	24,620,372
Common stock options	0	0	0
Denominator for diluted net loss per share	31,202,842	27,279,008	24,620,372