Net Income (Loss) Per Share - Calculations of Net Income (Loss) Per Share (Detail) - USD ($) $ / shares in Units, shares in Thousands, $ 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
Basic:
Net loss	$ (16,623)		$ 32,060		$ (24,896)		$ (6,170)		$ (18,728)		$ (18,650)		$ (29,097)		$ (22,097)		$ (15,629)	$ (88,572)
Basic (in shares)																	20,544	20,430
Net income (loss) per share (in dollars per share)	$ (0.81)	[1]	$ 1.56	[1]	$ (1.21)	[1]	$ (0.30)	[1]	$ (0.91)	[1]	$ (0.91)	[1]	$ (1.43)	[1]	$ (1.08)	[1]	$ (0.76)	$ (4.34)
Diluted:
Net loss	$ (16,623)		$ 32,060		$ (24,896)		$ (6,170)		$ (18,728)		$ (18,650)		$ (29,097)		$ (22,097)		$ (15,629)	$ (88,572)
Diluted (in shares)																	20,544	20,430
Net income (loss) per share (in dollars per share)	$ (0.81)	[1]	$ 1.56	[1]	$ (1.21)	[1]	$ (0.30)	[1]	$ (0.91)	[1]	$ (0.91)	[1]	$ (1.43)	[1]	$ (1.08)	[1]	$ (0.76)	$ (4.34)

[1] Per share amounts for the quarters have each been calculated separately. Accordingly, quarterly amounts may not add to the annual amounts because of differences in the average common shares outstanding during each period. Additionally, in regard to diluted per share amounts only, quarterly amounts may not add to the annual amounts because of the inclusion of the effect of potentially dilutive securities only in the periods in which such effect would have been dilutive, and because of the adjustments to net income (loss) for the dilutive effect of partner company common stock equivalents and convertible securities.