Stock Based Compensation - Additional Information (Detail) (USD $)	12 Months Ended
	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Total stock-based compensation expense	$ 3,159,000	$ 3,466,000	$ 3,290,000
Common stock reserved	19,700,000
Shares reserved for future issuance	16,796,884
Maximum term of options granted	10 years
Number of options to be granted to any one individual	1,300,000
Percentage of options vested	25.00%
Number of underlying shares exercisable	6,037,169	5,212,897	5,031,587
Weighted average exercise price of options exercisable	$ 2.89	$ 3.49	$ 3.55
Weighted average remaining contractual term of options outstanding	6 years 8 months 12 days
Weighted average remaining contractual term of options exercisable	6 years
Weighted average remaining contractual term of vested and unvested options expected to vest	6 years 7 months 6 days
Aggregate intrinsic value of options outstanding	0
Aggregate intrinsic value of options exercisable	0
Unrecognized compensation cost related to unvested options	1,000,000
Unvested stock-based awards expected to be recognized, weighted-average period	1 year 2 months 23 days
Weighted average grant-date fair value of options granted	$ 0.73	$ 1.31	$ 1.88
Total intrinsic value of options exercised	0	500,000	100,000
Shares available for grant	6,915,536
Aggregate grant-date fair value of RSUs vested	800,000	1,000,000	1,100,000
Common stock underlying RSUs that were fully vested	326,850
Restricted stock units (RSUs) [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Total stock-based compensation expense	1,100,000	1,000,000	800,000
Percentage of options vested	25.00%
Aggregate RSUs granted	828,000	409,189	258,456
Unvested stock-based awards expected to be recognized, weighted-average period	1 year 2 months 16 days
Aggregate grant-date fair value of RSUs granted	1,200,000	1,000,000	900,000
Total unrecognized compensation cost related to unvested RSUs	$ 600,000