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Lines of Credit and Long-Term Debt - Components of Interest Expense (Details) - 3.5% Convertible Notes Due 2023 - USD ($)
$ in Thousands
12 Months Ended
Dec. 31, 2020
Dec. 31, 2019
Dec. 31, 2018
Debt Instrument [Line Items]      
Cash interest expense $ 7,000 $ 7,000 $ 3,462
Non-cash interest expense 10,587 9,459 4,378
Total interest expense $ 17,587 $ 16,459 $ 7,840