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Stockholders' Equity (Narrative) (Details)
 1 Months Ended 6 Months Ended 12 Months Ended
Feb. 28, 2018
USD ($)
$ / shares
shares
Jan. 31, 2018
shares
Jun. 30, 2017
$ / shares
shares
Jun. 30, 2017
$ / shares
Dec. 31, 2017
USD ($)
item
$ / shares
shares
Dec. 31, 2016
USD ($)
$ / shares
shares
Dec. 31, 2015
USD ($)
$ / shares
shares
Aug. 31, 2017
USD ($)
Feb. 28, 2017
USD ($)
Feb. 28, 2013
$ / shares
shares
Oct. 31, 2010
$ / shares
shares
Stockholders' Equity [Line Items]                      
Common Stock, shares authorized | shares         1,000,000,000 1,000,000,000         1,000,000,000
Common Stock, par value | $ / shares         $ 0.001 $ 0.001         $ 0.001
Preferred Stock, shares authorized | shares         50,000,000 50,000,000         50,000,000
Preferred Stock, par value | $ / shares         $ 0.001 $ 0.001         $ 0.001
Preferred Stock, shares issued | shares         0 0          
Preferred Stock, shares outstanding | shares         0 0          
Common stock, dividends declared | $ / shares         $ 0            
Voting right of common stock share holders         one            
Voting right of common stock share holdes, number of votes | item         1            
Amount of common stock, preferred stock, depositiry shares, warrants, units or debt securities available in future public offerings               $ 150,000,000      
Number of shares of common stock called by issuance of warrants | shares           4,200,000       5,500,000  
Aggregate exercise price of common stock called by issuance of warrants | $ / shares                   $ 2.63  
Number of shares of common stock called by issuance of warrants, outstanding | shares         0 0          
Number of equity compensation plans | item         3            
Aggregate intrinsic value outstanding         $ 1,700,000            
Aggregate intrinsic value exercisable options         $ 1,600,000            
Stock price | $ / shares         $ 2.64            
Weighted average remaining contractual life for exercisable         5 years 5 months 5 days            
Vested and expected to vest, outstanding | shares         22,324,228            
Vested and expected to vest, aggregate intrinsic value         $ 1,600,000            
Vested and expected to vest, weighted average exercise price | $ / shares         $ 6.04            
Vested and expected to vest, weighted average remaining contractual life         6 years 2 months 23 days            
Total intrinsic value of options exercised         $ 1,700,000 $ 4,700,000 $ 3,500,000        
Fair value of options vested         6,400,000 20,700,000 7,300,000        
Stock-based compensation cost capitalized in inventory         441,000 389,000          
Stock-based compensation         $ 20,352,000 $ 19,562,000 $ 13,840,000        
2010 Plan [Member]                      
Stockholders' Equity [Line Items]                      
Stock option grant, term         10 years            
Stock option grant, vest         4 years            
Stock option grant, vest rate upon first anniversary         25.00%            
Stock option grant, vest rate per month thereafter         2.08%            
2010 Director Plan [Member]                      
Stockholders' Equity [Line Items]                      
Stock option grant, term         10 years            
Stock option grant, vest rate upon first anniversary         33.00%            
Stock option grant, vest rate per month thereafter         2.78%            
2010 Director Plan [Member] | Minimum [Member]                      
Stockholders' Equity [Line Items]                      
Stock option grant, vest         1 year            
2010 Director Plan [Member] | Maximum [Member]                      
Stockholders' Equity [Line Items]                      
Stock option grant, vest         3 years            
2010 Plan and 2010 Director Plan [Member]                      
Stockholders' Equity [Line Items]                      
Common stock reserved for issuance | shares         6,800,000            
ESPP [Member]                      
Stockholders' Equity [Line Items]                      
Common stock reserved for issuance | shares         1.3            
Percentage of outstanding common stock used to determine annual plan increase         2.00%            
Number of purchase periods | item         4            
Purchase period of ESPP         6 months            
Percentage of fair market value at which stock can be purchased         85.00%            
Weighted average fair value at grant date | $ / shares         $ 2.28 $ 4.21 $ 2.20        
Stock-based compensation         $ 3,100,000 $ 3,700,000 $ 3,600,000        
Common stock purchased under plan | shares         1,300,000 1,300,000 1,100,000        
Cash received from option exercises         $ 5,300,000 $ 5,200,000 $ 4,400,000        
At the Market Offering [Member]                      
Stockholders' Equity [Line Items]                      
Common stock offering price shares                 $ 60,000,000    
Aggregate offering price of shares sold         $ 11,865,000 $ 58,200,000 29,100,000        
Shares issued | shares         3,200,000 6,500,000          
Average common stock price per share | $ / shares         $ 3.86 $ 9.19          
Proceeds from issuance of common stock from offering, net of issuance costs         $ 11,865,000 $ 58,200,000 29,100,000        
Commissions       3.00%              
At the Market Offering [Member] | Common Stock [Member]                      
Stockholders' Equity [Line Items]                      
Aggregate offering price of shares sold         $ 3,000 $ 7,000 $ 4,000        
Shares issued | shares         3,171,000 6,526,000 4,075,000        
Underwritten Public Equity Offering [Member]                      
Stockholders' Equity [Line Items]                      
Aggregate offering price of shares sold         $ 52,530,000            
Shares issued | shares     17,700,000                
Proceeds from issuance of common stock from offering, net of issuance costs         52,530,000            
Commissions     4.00%                
Price per share | $ / shares     $ 3.10 $ 3.10              
Underwritten Public Equity Offering [Member] | Common Stock [Member]                      
Stockholders' Equity [Line Items]                      
Aggregate offering price of shares sold         $ 18,000            
Shares issued | shares         17,732,000            
Stock Options [Member]                      
Stockholders' Equity [Line Items]                      
Weighted average remaining contractual life         2 years 6 months            
Weighted average fair value at grant date | $ / shares         $ 3.08 $ 5.47 $ 4.75        
Stock-based compensation         $ 17,200,000 $ 15,700,000 $ 10,200,000        
Unrecognized compensation costs         22,500,000            
Cash received from option exercises         $ 3,600,000 $ 2,500,000 $ 3,000,000        
Subsequent Event [Member] | 2010 Plan [Member]                      
Stockholders' Equity [Line Items]                      
Additional common stock reserved for issuance | shares   7,000,000                  
Subsequent Event [Member] | ESPP [Member]                      
Stockholders' Equity [Line Items]                      
Additional common stock reserved for issuance | shares   2,300,000                  
Subsequent Event [Member] | Underwriting Agreement [Member]                      
Stockholders' Equity [Line Items]                      
Shares issued | shares 14,375,000                    
Proceeds from issuance of common stock from offering, net of issuance costs $ 32,800,000                    
Commissions 4.00%                    
Price per share | $ / shares $ 2.40                    
Notes [Member]                      
Stockholders' Equity [Line Items]                      
Holders may elect to receive net proceeds from financing activities, percentage         25.00%            
Holders may elect to receive net proceeds from financing activities as partial payment of notes         25.00%