Computation of Basic and Diluted Loss per Share under Two-Class Method (Detail) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended				6 Months Ended
	Jun. 30, 2019	Mar. 31, 2019	Jun. 30, 2018	Mar. 31, 2018	Jun. 30, 2019	Jun. 30, 2018
Numerator for basic and diluted loss per share:
Net loss	$ (16,205)	$ (19,994)	$ (14,812)	$ (22,272)	$ (36,199)	$ (37,084)
Effect of reallocating undistributed earnings of participating securities	0		0		0	0
Net loss available under the two-class method	$ (16,205)		$ (14,812)		$ (36,199)	$ (37,084)
Denominator:
Denominator for basic loss per share--weighted-average shares	28,615,006		26,930,914		28,043,040	26,860,146
Effect of dilutive potential common shares	0		0		0	0
Denominator for diluted loss per share--adjusted weighted-average shares	28,615,006		26,930,914		28,043,040	26,860,146
Basic loss per share	$ (0.57)		$ (0.55)		$ (1.29)	$ (1.38)
Diluted loss per share	$ (0.57)		$ (0.55)		$ (1.29)	$ (1.38)