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Preferred Stock and Stockholders' Equity (Narrative) (Details)
1 Months Ended 12 Months Ended
Feb. 07, 2018
shares
Feb. 10, 2017
shares
Mar. 31, 2015
USD ($)
shares
Dec. 31, 2017
USD ($)
item
$ / shares
shares
Dec. 31, 2016
USD ($)
item
$ / shares
shares
Dec. 31, 2015
USD ($)
shares
Dec. 31, 2014
shares
Dec. 31, 2012
shares
Shareholders Equity [Line Items]                
Preferred stock, shares authorized       10,000,000 10,000,000      
Preferred stock, par value | $ / shares       $ 0.001 $ 0.001      
Preferred stock, shares outstanding       0 0      
Proceeds from exercise of warrants, net of issuance costs | $       $ 0 $ 0 $ 17,088,000    
Increase in shares authorized for grant       4,508,000 4,386,000 4,056,000    
Number of stock option plans | item         2      
Shares available for future grants       7,630,000 7,920,000 8,070,000 7,546,000  
Total intrinsic value of options exercised | $       $ 22,400,000 $ 14,800,000 $ 5,500,000    
Fair value of options vested | $       12,300,000 7,000,000 5,400,000    
Non-cash stock-based compensation expense | $       13,361,000 7,058,000 6,013,000    
Non-employee                
Shareholders Equity [Line Items]                
Non-cash stock-based compensation expense | $       $ 31,000 56,000 87,000    
Number of awards outstanding | item       0        
Employee and Director Stock Option                
Shareholders Equity [Line Items]                
Non-cash stock-based compensation expense | $       $ 13,400,000 $ 7,100,000 $ 6,000,000    
Unrecognized compensation expense | $       $ 33,700,000        
Unrecognized compensation expense, weighted-average vesting period       2 years 10 months 13 days        
2012 Equity Incentive Award Plan                
Shareholders Equity [Line Items]                
Increase in shares authorized for grant   4,500,000            
Increase in shares available for issuance based on percentage of common stock outstanding       4.00% 4.00%      
Shares available for future grants               5,300,000
2012 Equity Incentive Award Plan | Minimum                
Shareholders Equity [Line Items]                
Exercise price as percentage of fair value of common stock       100.00%        
Stock options, vesting period       1 year        
2012 Equity Incentive Award Plan | Maximum                
Shareholders Equity [Line Items]                
Stock options, vesting period       4 years        
2012 Equity Incentive Award Plan | Subsequent Event                
Shareholders Equity [Line Items]                
Increase in shares authorized for grant 4,600,000              
2004 Equity Incentive Plan                
Shareholders Equity [Line Items]                
Shares available for future grants               0
2004 Equity Incentive Plan | Minimum                
Shareholders Equity [Line Items]                
Exercise price as percentage of fair value of common stock       100.00%        
Stock options, vesting period       1 year        
2004 Equity Incentive Plan | Maximum                
Shareholders Equity [Line Items]                
Stock options, vesting period       5 years        
Private Placement                
Shareholders Equity [Line Items]                
Shares issued upon exercise of warrants     6,200,000          
Number of private placement transactions | item       2        
Proceeds from exercise of warrants, net of issuance costs | $     $ 17,100,000          
Venture Capital Funds, Trusts and Other Entities | Private Placement                
Shareholders Equity [Line Items]                
Shares issued upon exercise of warrants     3,100,000          
Proceeds from exercise of warrants, net of issuance costs | $     $ 5,900,000