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Convertible Senior Notes, Net (Details)
2 Months Ended 3 Months Ended 6 Months Ended 12 Months Ended
Mar. 15, 2019
USD ($)
Aug. 02, 2018
USD ($)
shares
Sep. 30, 2018
USD ($)
shares
Jun. 30, 2019
USD ($)
Mar. 31, 2019
USD ($)
shares
Dec. 31, 2018
USD ($)
Jun. 30, 2018
USD ($)
Jun. 30, 2019
USD ($)
Jun. 30, 2018
USD ($)
Dec. 31, 2018
USD ($)
offering
Dec. 31, 2017
USD ($)
Dec. 31, 2014
USD ($)
offering
Oct. 05, 2018
USD ($)
Debt Instrument [Line Items]                          
Cash payment for debt redemption               $ 704,000 $ 0        
Conversions of convertible senior notes for common stock         $ 33,778,000                
Convertible senior notes, net       $ 559,619,000   $ 592,000,000   559,619,000   $ 592,000,000      
Common Stock                          
Debt Instrument [Line Items]                          
Conversions of convertible senior notes for common stock (in shares) | shares         1,967,361                
Conversions of convertible senior notes for common stock         $ 20,000                
Additional Paid In Capital                          
Debt Instrument [Line Items]                          
Conversions of convertible senior notes for common stock         $ 33,758,000                
Convertible Debt                          
Debt Instrument [Line Items]                          
Principal Amount       575,000,000       575,000,000          
Interest expense       7,200,000     $ 7,600,000 14,800,000 15,200,000        
Non-cash interest expense       1,400,000     $ 1,800,000 3,100,000 $ 3,700,000        
Convertible Debt | 2019 and 2022 Notes [Member] | Additional Paid In Capital                          
Debt Instrument [Line Items]                          
Conversions of convertible senior notes for common stock               15,400,000          
Convertible Debt | 2019 Notes                          
Debt Instrument [Line Items]                          
Number of debt offerings issued | offering                       2  
Principal Amount                       $ 254,800,000  
Coupon Rate                       5.50%  
Proceeds from issuance of convertible senior notes                       $ 248,600,000  
Principal amount redeemed   $ 206,200,000 $ 47,900,000                    
Cash payment for debt redemption $ 700,000 $ 39,300,000 $ 200,000                    
Loss on early extinguishment of debt       0       0   $ 2,600,000      
Convertible Debt | 2019 Notes | Common Stock                          
Debt Instrument [Line Items]                          
Conversions of convertible senior notes for common stock (in shares) | shares   10,020,328 2,775,509   1,967,361                
Conversions of convertible senior notes for common stock         $ 33,800,000                
Convertible Debt | 2019 Notes | Additional Paid In Capital                          
Debt Instrument [Line Items]                          
Conversions of convertible senior notes for common stock   $ 166,000,000.0 $ 13,900,000                    
Convertible Debt | 2022 Notes                          
Debt Instrument [Line Items]                          
Number of debt offerings issued | offering                   2      
Principal Amount                     $ 345,000,000.0    
Coupon Rate                     4.75%    
Proceeds from issuance of convertible senior notes                     $ 337,500,000    
Convertible senior notes, net       336,500,000       336,500,000          
Unamortized discount       8,500,000       8,500,000          
Convertible Debt | 2022 Notes | Additional Paid In Capital                          
Debt Instrument [Line Items]                          
Conversions of convertible senior notes for common stock               11,000,000.0          
Convertible Debt | 2023 Notes                          
Debt Instrument [Line Items]                          
Principal Amount           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       223,100,000       223,100,000          
Unamortized discount       $ 6,900,000       6,900,000          
Convertible Debt | 2023 Notes | Additional Paid In Capital                          
Debt Instrument [Line Items]                          
Conversions of convertible senior notes for common stock               $ 4,400,000