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Convertible Notes and Credit Facility - Narrative (Details)
1 Months Ended 3 Months Ended 9 Months Ended
Feb. 28, 2019
USD ($)
shares
Aug. 31, 2018
USD ($)
Mar. 31, 2018
USD ($)
Feb. 28, 2014
USD ($)
$ / shares
Sep. 30, 2019
USD ($)
$ / shares
Mar. 31, 2019
USD ($)
shares
Sep. 30, 2018
USD ($)
Sep. 30, 2019
USD ($)
$ / shares
Sep. 30, 2018
USD ($)
Dec. 31, 2018
USD ($)
$ / shares
Jun. 30, 2018
USD ($)
Mar. 12, 2018
USD ($)
Debt Instrument [Line Items]                        
Common stock, par value (usd per share) | $ / shares         $ 0.001     $ 0.001   $ 0.001    
Debt offering-related expenses               $ 128,000 $ 2,779,000      
Gain (loss) on extinguishment of convertible notes         $ 0   $ 0 (9,000,000) $ 0      
Convertible Debt                        
Debt Instrument [Line Items]                        
Aggregate principal amount $ 150,000,000.0       51,250,000     51,250,000   $ 201,249,000    
Convertible Debt | Trustee                        
Debt Instrument [Line Items]                        
Maximum borrowing capacity 11,900,000         $ 138,100,000            
Convertible Debt | 2014 Notes                        
Debt Instrument [Line Items]                        
Aggregate principal amount       $ 201,300,000 $ 51,250,000     $ 51,250,000   51,250,000    
Interest rate on notes       2.75% 2.75%     2.75%        
Initial conversion rate of notes       17.8750                
Common stock, par value (usd per share) | $ / shares       $ 0.001                
Initial conversion price of stock (usd per share) | $ / shares       $ 55.94                
Debt instrument redemption price when undergo fundamental change (in percent)       100.00%                
Proceeds from issuance of convertible notes, net       $ 195,200,000                
Debt offering-related expenses       1,100,000                
Underwriting discount       $ 6,000,000.0 $ 1,182,000     $ 1,182,000   1,232,000    
Aggregate principal loan outstanding         51,250,000     51,250,000   51,250,000    
Convertible Debt | 2014 Notes | Period One                        
Debt Instrument [Line Items]                        
Debt redemption conditioned upon common stock value exceeding a percentage of the conversion price       130.00%                
Convertible Debt | 2014 Notes | Period Two                        
Debt Instrument [Line Items]                        
Debt instrument redemption price (as percent)       100.00%                
Convertible Debt | 2014 Notes | Period Three                        
Debt Instrument [Line Items]                        
Debt instrument redemption price (as percent)       100.00%                
Convertible Debt | 2018 Notes                        
Debt Instrument [Line Items]                        
Aggregate principal amount     $ 150,000,000.0   $ 0     $ 0   149,999,000   $ 150,000,000.0
Interest rate on notes     2.75%   2.75%     2.75%     2.75%  
Initial conversion rate of notes     126.9438                  
Common stock, par value (usd per share) | $ / shares         $ 0.001     $ 0.001        
Initial conversion price of stock (usd per share) | $ / shares         $ 7.88     $ 7.88        
Debt redemption conditioned upon common stock value exceeding a percentage of the conversion price     110.00%                  
Debt instrument redemption price when undergo fundamental change (in percent)     100.00%                  
Underwriting discount         $ 0     $ 0   29,558,000    
Aggregate principal loan outstanding     $ 150,000,000.0   0     0   $ 149,999,000    
Fair value of debt                       $ 145,500,000
Gain (loss) on extinguishment of convertible notes     $ 100,000                  
Debt instrument, redemption price, percentage of principal amount redeemed     120.00%                  
Value of conversion option recorded in additional paid in capital     $ 29,300,000                  
Offering related costs                     $ 2,800,000  
Debt issuance costs         2,200,000     2,200,000        
Equity issuance costs               600,000        
Maximum borrowing capacity     $ 51,300,000                  
Convertible Debt | 2018 Notes | Trustee                        
Debt Instrument [Line Items]                        
Gain (loss) on extinguishment of convertible notes (9,000,000.0)                      
Secured Debt | Revolving Credit Facility | Silicon Valley Bank                        
Debt Instrument [Line Items]                        
Interest rate on notes   5.50%                    
Debt issuance costs   $ 335,000                    
Maximum borrowing capacity   $ 15,000,000.0                    
Maximum borrowing capacity, percentage of eligible receivables   85.00%                    
Maximum borrowing capacity, percentage of eligible inventory   50.00%                    
Availability under the credit facility         9,000,000.0     9,000,000.0        
Outstanding line of credit         $ 0     $ 0        
Unused borrowing capacity fee, percent   0.75%                    
Commitment fee amount   $ 225,000                    
Increase in interest due to default (as percent)   5.00%                    
Secured Debt | Revolving Credit Facility | Silicon Valley Bank | Prime Rate                        
Debt Instrument [Line Items]                        
Basis spread on variable rate   0.50%                    
Secured Debt | Period One | Revolving Credit Facility | Silicon Valley Bank                        
Debt Instrument [Line Items]                        
Percent of prepayment fee   2.00%                    
Secured Debt | Period Two | Revolving Credit Facility | Silicon Valley Bank                        
Debt Instrument [Line Items]                        
Percent of prepayment fee   1.00%                    
Common Stock | Convertible Debt                        
Debt Instrument [Line Items]                        
Net proceeds from issuance of common stock $ 133,300,000                      
Common Stock | Convertible Debt | 2014 Notes                        
Debt Instrument [Line Items]                        
Gain (loss) on extinguishment of convertible notes           $ 9,000,000.0            
Conversion of debt into common stock (in shares) | shares 19,500,000         19,500,000