Computation of Basic and Diluted Loss per Share under Two-Class Method (Detail) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2018	Dec. 31, 2017
Numerator for basic and diluted loss per share:
Net loss	$ (21,613)	$ (16,736)	$ (14,812)	$ (22,272)	$ (17,384)	$ (178,465)	$ (24,822)	$ (32,444)	$ (75,433)	$ (253,116)
Effect of reallocating undistributed earnings of participating securities									0	0
Net loss available under the two-class method									$ (75,433)	$ (253,116)
Denominator:
Denominator for basic loss per share—weighted-average shares									27,015,994	26,664,247
Effect of dilutive potential common shares									0	0
Denominator for diluted loss per share—adjusted weighted-average shares									27,015,994	26,664,247
Basic loss per share	$ (0.80)	$ (0.62)	$ (0.55)	$ (0.83)	$ (0.65)	$ (6.69)	$ (0.93)	$ (1.22)	$ (2.79)	$ (9.49)
Diluted loss per share	$ (0.80)	$ (0.62)	$ (0.55)	$ (0.83)	$ (0.65)	$ (6.69)	$ (0.93)	$ (1.22)	$ (2.79)	$ (9.49)