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Convertible Senior Notes, Net (Details)
 2 Months Ended 3 Months Ended 9 Months Ended 12 Months Ended
Aug. 02, 2018
USD ($)
shares
Aug. 18, 2014
USD ($)
Mar. 17, 2014
USD ($)
Sep. 30, 2018
USD ($)
$ / shares
shares
Sep. 30, 2018
USD ($)
$ / shares
shares
Sep. 30, 2017
USD ($)
Sep. 30, 2018
USD ($)
$ / shares
shares
Sep. 30, 2017
USD ($)
Dec. 31, 2017
USD ($)
offering
Dec. 31, 2014
USD ($)
offering
Debt Instrument [Line Items]                    
Proceeds from issuance of convertible senior notes             $ 0 $ 227,700,000    
Cash payment for debt redemption             40,461,000 0    
Loss on early extinguishment of debt         $ 2,573,000 $ 0 2,573,000 0    
Convertible senior notes, net       $ 370,174,000 $ 370,174,000   $ 370,174,000   $ 584,897,000  
Share price (in dollars per share) | $ / shares       $ 18.87 $ 18.87   $ 18.87      
Exchange of convertible senior notes for common stock             $ 178,566,000      
2019 Notes [Member]                    
Debt Instrument [Line Items]                    
Loss on early extinguishment of debt         $ 2,600,000 0 2,600,000 0    
Convertible Debt [Member]                    
Debt Instrument [Line Items]                    
Principal amount       $ 380,932,000 380,932,000   380,932,000      
Conversion rate basis, principal amount             1,000      
Convertible Debt [Member] | 2019 Notes [Member]                    
Debt Instrument [Line Items]                    
Number of debt offerings issued | offering                   2
Principal amount     $ 35,932,000             $ 254,800,000.0
Coupon rate     5.50%             5.50%
Proceeds from issuance of convertible senior notes                   $ 248,600,000
Principal amount redeemed $ 206,200,000     12,600,000 218,800,000          
Cash payment for debt redemption $ 39,300,000     200,000            
Convertible senior notes, net       35,500,000 35,500,000   35,500,000      
Unamortized discount       500,000 500,000   500,000      
Effective rate     6.50%              
Conversion rate     0.0580421              
Remaining period of amortization (years)     5 months 12 days              
Interest expense on debt         5,500,000 4,700,000 20,700,000 11,700,000    
Additional non-cash interest expense         1,200,000 $ 1,200,000 4,900,000 $ 3,000,000    
Convertible Debt [Member] | 2022 Notes [Member]                    
Debt Instrument [Line Items]                    
Number of debt offerings issued | offering                 2  
Principal amount   $ 345,000,000             $ 345,000,000.0  
Coupon rate   4.75%             4.75%  
Proceeds from issuance of convertible senior notes                 $ 337,500,000  
Convertible senior notes, net       334,700,000 334,700,000   334,700,000      
Unamortized discount       $ 10,300,000 $ 10,300,000   $ 10,300,000      
Effective rate   5.61%                
Conversion rate   0.0502260                
Remaining period of amortization (years)   3 years 10 months 24 days                
Common Stock [Member]                    
Debt Instrument [Line Items]                    
Exchange of convertible senior notes for common stock (in shares) | shares             10,744,577      
Exchange of convertible senior notes for common stock             $ 107,000      
Common Stock [Member] | Convertible Debt [Member] | 2019 Notes [Member]                    
Debt Instrument [Line Items]                    
Exchange of convertible senior notes for common stock (in shares) | shares 10,020,328     724,250 10,744,577          
Additional Paid In Capital [Member]                    
Debt Instrument [Line Items]                    
Exchange of convertible senior notes for common stock             178,459,000      
Additional Paid In Capital [Member] | Convertible Debt [Member] | 2019 and 2022 Notes [Member]                    
Debt Instrument [Line Items]                    
Exchange of convertible senior notes for common stock             22,400,000      
Additional Paid In Capital [Member] | Convertible Debt [Member] | 2019 Notes [Member]                    
Debt Instrument [Line Items]                    
Exchange of convertible senior notes for common stock $ 166,000,000     $ 12,400,000     11,400,000      
Additional Paid In Capital [Member] | Convertible Debt [Member] | 2022 Notes [Member]                    
Debt Instrument [Line Items]                    
Exchange of convertible senior notes for common stock             $ 11,000,000