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Debt - Convertible Debt (Details)
 1 Months Ended 2 Months Ended 3 Months Ended 12 Months Ended
Feb. 16, 2018
shares
Feb. 15, 2018
USD ($)
shares
Jul. 18, 2017
shares
Sep. 30, 2018
USD ($)
Dec. 31, 2016
USD ($)
day
$ / shares
shares
Feb. 28, 2019
USD ($)
shares
Sep. 30, 2018
USD ($)
Mar. 31, 2018
shares
Dec. 31, 2018
USD ($)
$ / shares
shares
Dec. 31, 2017
USD ($)
$ / shares
shares
Dec. 31, 2016
USD ($)
$ / shares
Jun. 07, 2018
USD ($)
shares
Convertible Debt                        
Proceeds for capped call confirmations                 $ 0 $ 0 $ 13,450,000  
Common stock, par value (in dollars per share) | $ / shares                 $ 0.01 $ 0.01    
Common stock issued from underwriting agreement (in shares) | shares     21,122,449         20,239,839        
Derivative liability, noncurrent   $ 507,400,000                    
Change in fair value of derivatives                 $ 2,739,000 $ 0 0  
Common stock, shares authorized | shares                 500,000,000 250,000,000   500,000,000
Derivative liabilities                       $ 88,300,000
Interest expense                        
Common stock, par value (in dollars per share) | $ / shares                 $ 0.01 $ 0.01    
Common Stock Capped Call Confirmation                        
Convertible Debt                        
Derivative asset, noncurrent   $ 13,600,000                    
Interest expense                        
Capped call confirmations                 $ 13,500,000      
Capped call confirmations, initial cap price (in dollars per share) | $ / shares                 $ 7.20      
Capped call confirmations, premium percentage                 50.00%      
Common Stock                        
Convertible Debt                        
Common stock issued from underwriting agreement (in shares) | shares 885,000 19,354,839                    
Over-allotment Option | Common Stock                        
Convertible Debt                        
Common stock issued from underwriting agreement (in shares) | shares 885,000                      
Number of days granted underwriters for purchase of shares   30 days                    
Maximum number of additional shares granted to underwriters | shares 2,903,225                      
Subsequent Event                        
Interest expense                        
Exchange agreement, aggregate principal amount           $ 184,600,000            
Exchange agreement, shares authorized | shares           33,000,000            
Accrued and unpaid interest on exchange agreement           $ 700,000            
Proceeds from capped call confirmations, exchange of convertible debt           $ 14,600,000            
Convertible Notes | Common Stock Capped Call Confirmation                        
Convertible Debt                        
Other assets, current                       $ 2,400,000
Convertible Notes | 2016 Convertible Notes                        
Convertible Debt                        
Aggregate principal amount         $ 250,000,000           $ 250,000,000  
Net proceeds after deducting fees and estimated expenses         243,000,000              
Proceeds for capped call confirmations         $ 13,500,000              
Shares issued per increment of convertible debt | shares         40,849,675              
Debt conversion ratio (in shares)         0.1633987              
Conversion price (in dollars per share) | $ / shares         $ 6.12           $ 6.12  
Threshold consecutive trading days | day         30              
Threshold percentage         130.00%       130.00%      
Period following 5 consecutive trading day period, trading price less than 98% of product of sales price         5 years              
Period following issuance of stock rights options or warrants with price per share less than last reported sales price         60 days              
Period for measurement of sales price         10 days              
Excess of value per share for triggering event, percent         10.00%              
Liability component (in thousands)                        
Principal                 $ 400,000,000 $ 250,000,000    
Less: debt discount                 (74,145,000) (81,566,000)    
Less: deferred financing                 (4,115,000) (4,267,000)    
Net carrying value of the debt                 321,740,000 164,167,000    
Interest expense                        
Contractual interest expense                 11,426,000 7,528,000    
Amortization of deferred financing                 555,000 470,000    
Amortization of debt discount                 10,421,000 9,241,000    
Total                 $ 22,402,000 $ 17,239,000    
Effective interest rate of the liability component, convertible debt                 10.85% 10.85%    
Convertible Notes | 2016 Convertible Notes | Minimum                        
Convertible Debt                        
Threshold trading days         20 days              
Convertible Notes | 2016 Convertible Notes | Over-allotment Option | Private Placement Purchase Agreement                        
Convertible Debt                        
Aggregate principal amount         $ 25,000,000           $ 25,000,000  
Senior Notes                        
Interest expense                        
Effective interest rate of the liability component, convertible debt                 10.48% 0.00%    
Line of Credit                        
Debt Instruments                        
New credit facility borrowing capacity       $ 150,000,000.0     $ 150,000,000.0          
Debt instrument term       5 years     5 years          
Periodic payment, percentage       12.50%                
Proceeds from issuance of debt       $ 146,600,000                
Line of Credit | LIBOR                        
Debt Instruments                        
Basis spread on variable rate       7.50%     7.50%