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, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Numerator for basic and diluted loss per share:
Net loss	$ (17,384)	$ (178,465)	$ (24,822)	$ (32,444)	$ (15,197)	$ (19,950)	$ (20,296)	$ (24,684)	$ (253,116)	$ (80,127)	$ (109,544)
Effect of reallocating undistributed earnings of participating securities									0	0	0
Net loss available under the two-class method									$ (253,116)	$ (80,127)	$ (109,544)
Denominator:
Denominator for basic loss per share—weighted-average shares									26,664,247	24,377,839	22,999,318
Effect of dilutive potential common shares									0	0	0
Denominator for diluted loss per share—adjusted weighted-average shares									26,664,247	24,377,839	22,999,318
Basic loss per share	$ (0.65)	$ (6.69)	$ (0.93)	$ (1.22)	$ (0.57)	$ (0.81)	$ (0.88)	$ (1.07)	$ (9.49)	$ (3.29)	$ (4.76)
Diluted loss per share	$ (0.65)	$ (6.69)	$ (0.93)	$ (1.22)	$ (0.57)	$ (0.81)	$ (0.88)	$ (1.07)	$ (9.49)	$ (3.29)	$ (4.76)