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Debt - Convertible Debt (Details)
$ / shares in Units, $ in Thousands
 3 Months Ended
Mar. 15, 2018
shares
Feb. 16, 2018
shares
Feb. 15, 2018
USD ($)
shares
Dec. 21, 2016
USD ($)
Mar. 31, 2018
USD ($)
D
item
$ / shares
shares
Mar. 31, 2017
USD ($)
Dec. 31, 2017
USD ($)
Convertible Debt              
Maximum number of additional shares granted to underwriters | shares   2,903,225          
Change in fair value of derivatives         $ 4,861    
Additional paid-in capital         1,621,479   $ 1,400,758
Current derivative liability         $ 80,577    
Common stock conversion feature              
Convertible Debt              
Derivative liability     $ 507,400        
Additional paid-in capital     (507,400)        
Common Stock Capped Call Confirmations              
Convertible Debt              
Derivative asset     13,600        
Additional paid-in capital     $ 13,600        
Common Stock              
Convertible Debt              
Number of common stock shares issued in underwriting agreement | shares 885,000   19,354,839   20,239,839    
Convertible Notes | Common Stock Capped Call Confirmations              
Convertible Debt              
Other current asset         $ 2,200    
Convertible Notes | 2016 Convertible Notes              
Convertible Debt              
Aggregate principal amount       $ 250,000      
Net proceeds after deducting fees and estimated expenses       243,000      
Net proceeds from convertible notes to pay capped call transactions       13,500      
Number of common stock shares issued upon conversion | item         40,849,675    
Debt conversion ratio         163.3987    
Conversion price (in dollars per share) | $ / shares         $ 6.12    
Threshold percentage         130.00%    
Threshold trading days | D         20    
Threshold consecutive trading days | D         30    
Liability components of convertible notes              
Principal         $ 250,000   250,000
Less: debt discount         (79,095)   (81,566)
Less: deferred financing         (4,137)   (4,267)
Net carrying value of the debt         166,768   $ 164,167
Interest expense              
Contractual interest expense         1,887 $ 1,887  
Amortization of debt discount         2,472 2,261  
Amortization of deferred financing         129 142  
Total         $ 4,488 $ 4,290  
Over-allotment Option | Common Stock              
Convertible Debt              
Number of days in option period granted to underwriters     30 days        
Maximum number of additional shares granted to underwriters | shares     2,903,225        
Over-allotment Option | Private Placement Purchase Agreement | Convertible Notes | 2016 Convertible Notes              
Convertible Debt              
Aggregate principal amount       $ 25,000