Stock-Based Compensation - Summary of Stock Option Activity (Detail) - USD ($) $ / shares in Units, $ in Thousands	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Number of Shares
Outstanding, beginning (shares)	13,836,851	12,649,102	11,329,218
Granted (shares)	1,435,550	2,219,587	2,576,292
Exercised (shares)	(39,010)	(456,082)	(485,260)
Forfeited (shares)	(379,268)	(296,162)	(557,109)
Expired (shares)	(513,541)	(279,594)	(214,039)
Outstanding, ending (shares)	14,340,582	13,836,851	12,649,102
Vested (exercisable) (shares)	11,206,200
Unvested (unexercisable) (shares)	3,134,382
Weighted- Average Exercise Price/Share
Outstanding, beginning ($ per share)	$ 6.97	$ 7.12	$ 7.10
Granted ($ per share)	5.94	6.04	7.60
Exercised ($ per share)	5.18	4.45	4.77
Forfeited ($ per share)	7.21	8.06	9.65
Expired ($ per share)	7.26	9.05	10.70
Outstanding, ending ($ per share)	6.86	$ 6.97	$ 7.12
Vested (exercisable) ($ per share)	6.99
Unvested (unexercisable) ($ per share)	$ 6.37
Weighted-Average Remaining Contractual Term (Years), Outstanding	5 years 8 months 16 days
Weighted-Average Remaining Contractual Term (Years), Vested (exercisable)	4 years 10 months 12 days
Weighted-Average Remaining Contractual Term (Years), Unvested (unexercisable)	8 years 8 months 11 days
Stock options exercised, Intrinsic value	$ 50	$ 977	$ 1,629
Outstanding, Aggregate Intrinsic Value	2,848
Aggregate Intrinsic Value, Vested (exercisable)	2,829
Aggregate Intrinsic Value, Unvested (unexercisable)	$ 19