Stock Award Plans and Stock Based Compensation (Details) - USD ($) $ / shares in Units, $ in Thousands	12 Months Ended			36 Months Ended	44 Months Ended
	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2015	Dec. 31, 2015	May. 02, 2012
Stock award plans and stock-based compensation
Common stock, par value (in dollars per share)	$ 0.001	$ 0.001		$ 0.001	$ 0.001	$ 0.001
Stock-Based Compensation Expense
Forfeiture rate (as a percent)	5.00%	5.00%	5.00%
Stock-based compensation expense (in dollars)	$ 5,600	$ 4,800	$ 4,300
Accelerated compensation expenses	$ 1,100
Employee stock purchase plan
Stock award plans and stock-based compensation
Number of shares of common stock available for future grant	1,400,000			1,400,000	1,400,000
Employee Stock Purchase Plan
Purchase price as a percentage of the market value of a common stock on the day the stock is purchased	85.00%
Offering period over which compensation expense is amortized	6 months
Number of shares purchased under the plan	200,000	200,000	200,000
Stock-Based Compensation Expense
Stock-based compensation expense (in dollars)	$ 100	$ 100	$ 100
Employee stock purchase plan | Maximum
Employee Stock Purchase Plan
Payroll deductions as a percentage of employee's salary	10.00%
Restricted stock and restricted stock units
Shares/units
Outstanding at the beginning of the period (in shares)	103,000
Granted (in shares)	293,000
Vested (in shares)	(30,000)
Outstanding at the end of the period (in shares)	366,000	103,000		366,000	366,000
Weighted-Average Grant Date Fair Value per Share
Outstanding at the beginning of the period (in dollars per share)	$ 2.07
Granted (in dollars per share)	3.03
Vested (in dollars per share)	1.97
Outstanding at the end of the period (in dollars per share)	$ 2.85	$ 2.07		$ 2.85	$ 2.85
Stock Options
Estimated weighted-average assumptions
Weighted-average expected volatility (as a percent)			98.10%
Weighted-average expected term			4 years 8 months 12 days
Risk-free interest rate, minimum (as a percent)	1.27%	1.19%	0.70%
Risk-free interest rate, maximum (as a percent)	1.58%	1.67%	1.40%
Expected dividend yield (as a percent)	0.00%	0.00%	0.00%
Options
Outstanding at the beginning of the period (in shares)	21,704,000
Granted (in shares)	3,358,000
Exercised (in shares)	(3,036,000)
Canceled (in shares)	(101,000)
Expired (in shares)	(256,000)
Outstanding at the end of the period (in shares)	21,669,000	21,704,000		21,669,000	21,669,000
Exercisable (in shares)	13,462,000			13,462,000	13,462,000
Options Vested or Expected to Vest at the end of the period (in shares)	21,098,000			21,098,000	21,098,000
Weighted Average Exercise Price
Outstanding at the beginning of the period (in dollars per share)	$ 1.62
Granted (in dollars per share)	3.01
Exercised (in dollars per share)	1.29
Canceled (in dollars per share)	1.80
Expired (in dollars per share)	6.17
Outstanding at the end of the period (in dollars per share)	1.82	$ 1.62		$ 1.82	$ 1.82
Exercisable at the end of the period (in dollars per share)	1.56			1.56	1.56
Options Vested or Expected to Vest at the end of the period (in dollars per share)	$ 1.81			$ 1.81	$ 1.81
Weighted Average Remaining Contractual Term
Outstanding at the end of the period	4 years 9 months 29 days
Exercisable at the end of the period	4 years 4 months 17 days
Options Vested or Expected to Vest at the end of the period	4 years 10 months 13 days
Aggregate Intrinsic Value
Outstanding at the end of the period (in dollars)	$ 18,761			$ 18,761	$ 18,761
Exercisable at the end of the period (in dollars)	14,570			14,570	14,570
Options Vested or Expected to Vest at the end of the period (in dollars)	18,535			18,535	18,535
Additional disclosure
Total intrinsic value of options exercised (in dollars)	5,300	$ 2,400	$ 1,400
Total fair value of stock options vested (in dollars)	5,400	$ 4,700	$ 3,800
Total forfeiture adjusted unrecognized compensation cost (in dollars)	$ 8,500			$ 8,500	$ 8,500
Weighted-average period over which unrecognized compensation cost is expected to be recognized	2 years 8 months 23 days
Stock Options | Minimum
Estimated weighted-average assumptions
Weighted-average expected volatility (as a percent)	56.22%	60.59%
Weighted-average expected term	4 years 7 months 10 days	3 years 9 months
Stock Options | Maximum
Estimated weighted-average assumptions
Weighted-average expected volatility (as a percent)	64.70%	98.40%
Weighted-average expected term	4 years 8 months 16 days	4 years 8 months 16 days
Stock Options | Employees
Stock award plans and stock-based compensation
Vesting period	4 years
Period after termination to exercise awards that were vested	90 days
Period after termination to retiring employees to exercise vested awards	1 year
Stock Options | Non-employee members of Board of Directors
Stock award plans and stock-based compensation
Vesting period	6 months
Restricted Stock
Shares/units
Granted (in shares)	0	0	0
Vested (in shares)	0	0	0
2000 Stock Plan
Stock award plans and stock-based compensation
Awards granted (in shares)					0
Number of shares of common stock available for future grant	0			0	0
2000 Stock Plan | Stock Options
Stock award plans and stock-based compensation
Expiration period	10 years
2012 Equity Incentive Plan
Stock award plans and stock-based compensation
Number of shares of common stock originally reserved for future grant						11,100,000
Number of shares of common stock available for future grant	1,800,000			1,800,000	1,800,000
Estimated weighted-average assumptions
Weighted-average expected term	7 years
2012 Equity Incentive Plan | Stock Options
Stock award plans and stock-based compensation
Expiration period	7 years
2012 Equity Incentive Plan | Restricted Stock
Shares/units
Granted (in shares)				0