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Convertible Senior Notes (Summary of Interest Expense for Contractual Interest and Accretion of Debt Discount) (Details) - Convertible Senior Notes [Member] - USD ($)
$ in Thousands
3 Months Ended 6 Months Ended
Jun. 30, 2019
Jun. 30, 2018
Jun. 30, 2019
Jun. 30, 2018
2028 Convertible Senior Notes [Member]        
Debt Instrument [Line Items]        
Contractual coupon interest expense $ 4,400   $ 9,142  
Amortization of debt discount 5,968   12,285  
Amortization of debt issuance costs 117   239  
Total interest expense related to the Notes 10,485   21,666  
2019 Convertible Senior Notes [Member]        
Debt Instrument [Line Items]        
Contractual coupon interest expense 372 $ 1,857 745 $ 3,713
Amortization of debt discount 532 2,508 1,063 5,016
Amortization of debt issuance costs 88 399 176 798
Total interest expense related to the Notes $ 992 $ 4,764 $ 1,984 $ 9,527