Summary of Stock Options Outstanding (Detail) $ / shares in Units, $ in Thousands	12 Months Ended
	Dec. 31, 2017 USD ($) $ / shares shares
Number of Shares
Outstanding at January 1, 2017 | shares	5,530,256
Granted | shares	3,335,385
Exercised | shares	(5,300)
Forfeited | shares	(598,204)
Expired | shares	(1,172,697)
Outstanding at December 31, 2017 | shares	7,089,440
Vested and expected to vest at December 31, 2017 | shares	6,735,675
Exercisable at December 31, 2017 | shares	2,813,227
Weighted Average Exercise Price per Share
Outstanding at January 1, 2017 | $ / shares	$ 16.10
Granted | $ / shares	1.60
Exercised | $ / shares	4.55
Forfeited | $ / shares	3.17
Expired | $ / shares	22.21
Outstanding at December 31, 2017 | $ / shares	9.33
Vested and expected to vest at December 31, 2017 | $ / shares	9.70
Exercisable at December 31, 2017 | $ / shares	$ 19.52
Aggregate Intrinsic Value
Outstanding at December 31, 2017 | $	$ 2,484
Vested and expected to vest at December 31, 2017 | $	2,262
Exercisable at December 31, 2017 | $	$ 61