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Stock-Based Compensation - ESPP & Valuation (Details)
1 Months Ended 12 Months Ended
Aug. 31, 2015
USD ($)
shares
Feb. 28, 2015
USD ($)
shares
May. 31, 2015
shares
Jun. 30, 2010
USD ($)
item
shares
Dec. 31, 2015
$ / shares
shares
Dec. 31, 2014
Dec. 31, 2013
Options              
Stock-Based Compensation              
Outstanding options fully vested and expected to vest (in shares)         8,214,000    
Remaining contractual life of options fully vested and expected to vest         3 years 9 months 22 days    
Weighted average exercise price of options fully vested and expected to vest (in dollars per share) | $ / shares         $ 3.71    
Awards exercisable (in shares)         6,232,000    
Weighted average exercise price of awards exercisable (in dollars per share) | $ / shares         $ 3.97    
Assumptions used in estimation of fair value of the share-based payment awards on the grant date              
Risk-free interest rate, minimum (as a percent)         0.60% 1.30% 0.50%
Risk-free interest rate, maximum (as a percent)         1.70% 1.70% 1.70%
Volatility, minimum (as a percent)         55.70% 53.70% 57.70%
Volatility, maximum (as a percent)         59.30% 57.50% 62.90%
Dividend yield (as a percent)         0.00% 0.00% 0.00%
Options | Minimum              
Assumptions used in estimation of fair value of the share-based payment awards on the grant date              
Expected life         3 years 4 years 4 years
Options | Maximum              
Assumptions used in estimation of fair value of the share-based payment awards on the grant date              
Expected life         5 years 5 years 5 years
ESPP              
Employee Stock Purchase Plan              
Number of shares reserved for issuance       2,000,000      
Additional shares authorized     2,000,000        
Number of offering periods | item       2      
Offering period       6 months      
Purchase price to be paid by participants as a percentage of price per share either at the beginning or the end of each six-month offering period, whichever is less       85.00%      
Shares of common stock issued under the plan 268,000 339,000          
Aggregate purchase price (in dollars) | $ $ 339,000 $ 518,000          
Shares authorized and unissued         2,200,000    
ESPP | Maximum              
Employee Stock Purchase Plan              
Maximum amount of shares that eligible employee may purchase annually (in dollars) | $       $ 25,000