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Stockholders' Equity - Equity Incentive Plans (Details) - USD ($)
 12 Months Ended
Dec. 31, 2016
Dec. 31, 2015
Dec. 31, 2014
Stockholders' Equity      
Deferred compensation liability, noncurrent $ 1,500,000 $ 700,000  
Trading securities 1,500,000 700,000  
Income from deferred compensation investments 34,000 17,000  
Unrealized gain/(loss) $ 32,000 $ 50,000  
Fair value weighted-average assumptions:      
Minimum stock price volatility (as a percent) 75.00%    
Common stock options      
Fair value weighted-average assumptions:      
Maximum term of option 10 years    
Expected stock price volatility (as a percent) 81.30% 75.90% 81.30%
Risk free interest rate (as a percent) 1.50% 1.70% 1.90%
Expected life of options 6 years 3 months 6 years 3 months 6 years 3 months
Expected annual dividend per share (in dollars per share) $ 0.00 $ 0.00 $ 0.00
Granted (in dollars per share) $ 5.28 $ 7.51 $ 2.12
Number of Shares      
Balance at the beginning of the period (in shares) 11,729,200 10,020,700 9,041,100
Options granted (in shares) 5,114,100 3,917,200 2,993,100
Options exercised (in shares) (723,100) (2,070,300) (965,600)
Options forfeited (in shares) (622,700) (138,400) (1,047,900)
Balance at the end of the period (in shares) 15,497,500 11,729,200 10,020,700
Vested and unvested expected to vest as of the end of the period (in shares) 14,593,700    
Exercisable at the end of the period (in shares) 7,545,300    
Weighted Average Exercise Price      
Balance at the beginning of the period (in dollars per share) $ 7.11 $ 5.02 $ 5.65
Options granted (in dollars per share) 7.67 11.61 2.99
Options exercised (in dollars per share) 4.20 5.43 3.80
Options forfeited (in dollars per share) 8.62 7.76 5.76
Balance at the end of the period (in dollars per share) 7.37 $ 7.11 $ 5.02
Vested and unvested expected to vest as of the end of the period (in dollars per share) 7.31    
Exercisable at the end of the period (in dollars per share) $ 6.57    
Weighted Average Remaining Contractual Life      
Balance at the end of the period 7 years 3 months 18 days    
Vested and unvested expected to vest at the end of the period 7 years 2 months 12 days    
Exercisable at the end of the period 5 years 9 months 18 days    
Aggregate Intrinsic Value      
Aggregate intrinsic value of options outstanding (in dollars) $ 7,800    
Aggregate intrinsic value of options vested and unvested expected to vest (in dollars) 7,700    
Aggregate intrinsic value of options exercisable (in dollars) 5,800    
Aggregate intrinsic value of options exercised (in dollars) 2,600,000 $ 14,700,000 $ 2,800,000
Cash proceeds from stock options exercised (in dollars) 3,000,000 $ 11,200,000 $ 3,700,000
Total unrecognized compensation cost related to non-vested stock options granted (in dollars) $ 33,700,000    
Period of recognition unrecognized compensation costs (in years) 2 years 9 months 18 days    
Common stock options | 2002 Plan and 2007 Plan      
Fair value weighted-average assumptions:      
Percentage of options vest on the first year anniversary 25.00%    
Additional percentage of options that will vest each month after first anniversary 2.08%    
Percentage of shares vested that may be exercised in whole or part 100.00%    
Restricted stock units (RSUs)      
Number of Shares      
Balance at the beginning of the period (in shares)   955,000  
Balance at the end of the period (in shares)     955,000
Weighted Average Exercise Price      
Balance at the beginning of the period (in dollars per share)   $ 2.28  
Balance at the end of the period (in dollars per share)     $ 2.28
Restricted Stock Units, Number of Shares      
Non-vested units as of the beginning of the period (in shares) 478,500    
Granted (in shares) 582,700 366,000  
Vested (in shares) (281,900) (842,500)  
Forfeited (in shares) (34,900)    
Non-vested units as of the end of the period (in shares) 744,400 478,500  
Weighted Average Grant Date Fair Value      
Non-vested units as of the beginning of the period (in dollars per share) $ 10.38    
Granted (in dollars per share) 6.21 $ 12.63  
Vested (in dollars per share) 8.73    
Forfeited (in dollars per share) 7.71    
Non-vested units as of the end of the period (in dollars per share) $ 7.86 $ 10.38  
Weighted Average Remaining Years      
Non-vested units as of the end of the period 2 years 7 months 13 days    
Restricted stock units (RSUs) | Performance-Based RSU Agreement      
Weighted Average Remaining Years      
Units issued under the Plan 0    
Restricted stock      
Aggregate Intrinsic Value      
Total unrecognized compensation cost related to non-vested stock options granted (in dollars) $ 4,200,000    
Period of recognition unrecognized compensation costs (in years) 2 years 7 months 13 days    
Maximum | Common stock options | 2007 Plan and 2007 Director Plan      
Fair value weighted-average assumptions:      
Number of shares reserved for issuance 12,630,551