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Debt - Convertible Debt (Details)
1 Months Ended 3 Months Ended 9 Months Ended
Feb. 16, 2018
shares
Feb. 15, 2018
USD ($)
shares
Sep. 30, 2018
USD ($)
shares
Dec. 31, 2016
USD ($)
day
$ / shares
shares
Sep. 30, 2018
USD ($)
shares
Mar. 31, 2018
shares
Sep. 30, 2017
USD ($)
Sep. 30, 2018
USD ($)
shares
Sep. 30, 2017
USD ($)
Jun. 07, 2018
USD ($)
shares
Dec. 31, 2017
USD ($)
shares
Convertible Debt                      
Common stock issued from underwriting agreement (in shares) | shares           20,239,839          
Derivative liability, noncurrent   $ 507,400,000                  
Change in fair value of derivatives         $ 0   $ 0 $ (2,739,000) $ 163,000    
Common stock, shares authorized | shares     500,000,000   500,000,000     500,000,000   500,000,000 250,000,000
Derivative liabilities                   $ 88,300,000  
Common Stock Capped Call Confirmation                      
Convertible Debt                      
Derivative asset, noncurrent   $ 13,600,000                  
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                      
Number of days granted underwriters for purchase of shares   30 days                  
Maximum number of additional shares granted to underwriters | shares 2,903,225                    
Convertible Notes                      
Convertible Debt                      
Threshold percentage               130.00%      
Convertible Notes | Common Stock Capped Call Confirmation                      
Convertible Debt                      
Other assets, current                   $ 2,400,000  
Convertible Notes | Convertible Senior Notes 2016, Due 2023                      
Convertible Debt                      
Aggregate principle amount       $ 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              
Threshold consecutive trading days | day       30              
Debt Instruments [Abstract]                      
Principal     $ 400,000,000   $ 400,000,000     $ 400,000,000     $ 250,000,000
Debt discount     (76,946,000)   (76,946,000)     (76,946,000)     (81,566,000)
Less: deferred financing     (4,246,000)   (4,246,000)     (4,246,000)     (4,267,000)
Net carrying value of the debt     318,808,000   318,808,000     318,808,000     $ 164,167,000
Interest expense                      
Contractual interest expense         1,969,000     5,744,000      
Amortization of debt discount         2,609,000     7,620,000      
Amortization of deferred financing         137,000     399,000      
Total         4,715,000     13,763,000      
Convertible Notes | Convertible Senior Notes 2016, Due 2023 | Minimum                      
Convertible Debt                      
Threshold trading days       20 days              
Convertible Notes | Convertible Senior Notes 2016, Due 2023 | Over-allotment Option | Private Placement Purchase Agreement                      
Convertible Debt                      
Aggregate principle amount       $ 25,000,000              
Line of Credit                      
Debt Instruments                      
New credit facility borrowing capacity     $ 150,000,000   $ 150,000,000     $ 150,000,000      
Debt instrument term         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%