Stock-Based Compensation (Details) (USD $)	9 Months Ended	12 Months Ended
	Sep. 30, 2012	Dec. 31, 2011
Stock-Based Compensation
Maximum shares of common stock to be issued for grants (in shares)	7,400,000
Stock options
Shares
Options outstanding at the beginning of the period (in shares)	4,459,034
Granted (in shares)	1,115,700
Exercised (in shares)	(21,679)
Canceled (in shares)	(201,056)
Options outstanding at the end of the period (in shares)	5,351,999	4,459,034
Options vested and expected to vest at the end of the period (in shares)	5,273,241
Options exercisable at the end of the period (in shares)	3,190,776
Shares Available for Grant under the 2008 Plan (in shares)	3,380,865
Weighted Average Exercise Price Per Share
Options outstanding at the beginning of the period (in dollars per share)	$ 6.08
Granted (in dollars per share)	$ 5.84
Exercised (in dollars per share)	$ 2.61
Canceled (in dollars per share)	$ 8.22
Options outstanding at the end of the period (in dollars per share)	$ 5.97	$ 6.08
Options vested and expected to vest at the end of the period (in dollars per share)	$ 5.98
Options Exercisable at the end of the period (in dollars per share)	$ 6.77
Weighted Average Remaining Contractual Term (In Years)
Options outstanding at the beginning of the period	7 years 2 months 12 days	6 years 10 months 24 days
Options outstanding at the end of the period	7 years 2 months 12 days	6 years 10 months 24 days
Options vested and expected to vest at the end of the period	7 years 1 month 6 days
Options Exercisable at the end of the period	5 years 10 months 24 days
Additional information on options
Weighted average grant-date fair value (in dollars per share)	$ 3.67