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Convertible Senior Notes - Interest Expense Recognized (Details) - Senior Notes - USD ($)
$ in Thousands
3 Months Ended 6 Months Ended
Jun. 30, 2020
Jun. 30, 2019
Jun. 30, 2020
Jun. 30, 2019
0.125% Convertible Senior Notes Due 2025        
Debt Instrument [Line Items]        
Contractual interest expense $ 249 $ 251 $ 498 $ 265
Amortization of debt discount 8,842 8,914 17,683 9,424
Amortization of issuance costs 607 613 1,215 648
Total interest expense 9,698 9,778 19,396 10,337
0.25% Convertible Senior Notes Due 2023        
Debt Instrument [Line Items]        
Contractual interest expense 215 215 430 428
Amortization of debt discount 3,126 3,125 6,251 6,216
Amortization of issuance costs 371 368 743 737
Total interest expense $ 3,712 $ 3,708 $ 7,424 $ 7,381