Computation of Basic and Diluted Loss per Share under Two-Class Method (Detail) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended						9 Months Ended
	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Sep. 30, 2019	Sep. 30, 2018
Numerator for basic and diluted loss per share:
Net loss	$ (29,428)	$ (16,205)	$ (19,994)	$ (16,736)	$ (14,812)	$ (22,272)	$ (65,627)	$ (53,821)
Effect of reallocating undistributed earnings of participating securities	0			0			0	0
Net loss available under the two-class method	$ (29,428)			$ (16,736)			$ (65,627)	$ (53,821)
Denominator:
Denominator for basic loss per share--weighted-average shares	28,642,839			27,169,301			28,245,170	26,964,330
Effect of dilutive potential common shares	0			0			0	0
Denominator for diluted loss per share--adjusted weighted-average shares	28,642,839			27,169,301			28,245,170	26,964,330
Basic loss per share	$ (1.03)			$ (0.62)			$ (2.32)	$ (2.00)
Diluted loss per share	$ (1.03)			$ (0.62)			$ (2.32)	$ (2.00)