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Convertible Senior Notes, Net - Additional Information (Details)
3 Months Ended 5 Months Ended 12 Months Ended
Aug. 02, 2018
USD ($)
shares
Dec. 31, 2018
USD ($)
$ / shares
Sep. 30, 2018
USD ($)
Jun. 30, 2018
USD ($)
Mar. 31, 2018
USD ($)
Dec. 31, 2017
USD ($)
Sep. 30, 2017
USD ($)
Jun. 30, 2017
USD ($)
Mar. 31, 2017
USD ($)
Dec. 31, 2018
USD ($)
$ / shares
shares
Dec. 31, 2018
USD ($)
$ / shares
shares
Dec. 31, 2017
USD ($)
offering
Dec. 31, 2016
USD ($)
Dec. 31, 2014
USD ($)
offering
Oct. 05, 2018
USD ($)
Mar. 17, 2014
USD ($)
Debt Instrument [Line Items]                                
Proceeds from issuance of convertible senior notes                     $ 226,550,000 $ 343,275,000 $ 0      
Cash payment for debt redemption                     40,461,000 0 0      
Loss on early extinguishment of debt   $ 0 $ 2,573,000 $ 0 $ 0 $ 1,947,000 $ 0 $ 0 $ 0   2,573,000 1,947,000 0      
Convertible senior notes, net   $ 592,000,000       584,897,000       $ 592,000,000 $ 592,000,000 584,897,000        
Share price (in dollars per share) | $ / shares   $ 16.66               $ 16.66 $ 16.66          
Exchange of Notes for common stock                     $ 180,016,000          
2019 Notes [Member]                                
Debt Instrument [Line Items]                                
Loss on early extinguishment of debt                     2,600,000          
Convertible Debt [Member]                                
Debt Instrument [Line Items]                                
Principal Amount   $ 609,482,000               $ 609,482,000 609,482,000          
Interest expense on debt                     28,200,000 18,700,000 14,000,000      
Additional non-cash interest expense                     7,100,000 $ 4,700,000 $ 3,500,000      
Convertible Debt [Member] | 2019 Notes [Member]                                
Debt Instrument [Line Items]                                
Number of debt offerings issued | offering                           2    
Principal Amount                           $ 254,800,000.0   $ 34,482,000
Coupon Rate                           5.50%    
Proceeds from issuance of convertible senior notes                           $ 248,600,000    
Principal amount redeemed $ 206,200,000                 14,100,000 220,300,000          
Cash payment for debt redemption $ 39,300,000                 200,000            
Convertible senior notes, net   34,300,000               34,300,000 34,300,000          
Unamortized discount   200,000               200,000 200,000          
Convertible Debt [Member] | 2022 Notes [Member]                                
Debt Instrument [Line Items]                                
Number of debt offerings issued | offering                       2        
Principal Amount           $ 345,000,000.0           $ 345,000,000.0        
Coupon Rate           4.75%           4.75%        
Proceeds from issuance of convertible senior notes                       $ 337,500,000        
Convertible senior notes, net   335,300,000               335,300,000 335,300,000          
Unamortized discount   9,700,000               9,700,000 9,700,000          
Convertible Debt [Member] | 2023 Notes [Member]                                
Debt Instrument [Line Items]                                
Principal Amount   230,000,000.0               230,000,000.0 230,000,000.0       $ 230,000,000  
Coupon Rate                             5.375%  
Proceeds from issuance of convertible senior notes   223,700,000                            
Convertible senior notes, net   222,400,000               222,400,000 222,400,000          
Unamortized discount   7,600,000               $ 7,600,000 $ 7,600,000          
Common Stock [Member]                                
Debt Instrument [Line Items]                                
Exchange of convertible senior notes for common stock (in shares) | shares                     10,828,475          
Exchange of Notes for common stock                     $ 108,000          
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                 808,148            
Additional Paid In Capital [Member]                                
Debt Instrument [Line Items]                                
Exchange of Notes for common stock                     179,908,000          
Fair value of debt component of exchanged and converted Notes   $ 27,200,000               $ 27,200,000 27,200,000          
Additional Paid In Capital [Member] | Convertible Debt [Member] | 2019 and 2022 Notes [Member]                                
Debt Instrument [Line Items]                                
Exchange of Notes for common stock                     26,800,000          
Additional Paid In Capital [Member] | Convertible Debt [Member] | 2019 Notes [Member]                                
Debt Instrument [Line Items]                                
Exchange of Notes for common stock $ 166,000,000                 $ 13,900,000 11,400,000          
Additional Paid In Capital [Member] | Convertible Debt [Member] | 2022 Notes [Member]                                
Debt Instrument [Line Items]                                
Exchange of Notes for common stock                     11,000,000          
Additional Paid In Capital [Member] | Convertible Debt [Member] | 2023 Notes [Member]                                
Debt Instrument [Line Items]                                
Exchange of Notes for common stock                     $ 4,400,000