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Convertible Notes (Tables) | 12 Months Ended
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Dec. 31, 2018
Debt Disclosure [Abstract] |
Schedule of convertible notes | The Company’s convertible notes are shown in the following table.
(Dollars in thousands) | As of December 31, 2018 |  | As of December 31, 2017
1.375% Convertible Senior Notes due 2023 | $ | 172,500 |  |  | $ | 172,500 |
1.125% Convertible Senior Notes due 2018 | — |  |  | 81,207 |
Total principal amount of convertible notes | 172,500 |  |  | 253,707 |
Unamortized discount - 2023 Notes | (28,517 | ) |  | (34,506 | )
Unamortized discount - 2018 Notes | — |  |  | (2,547 | )
Unamortized debt issuance costs - 2023 Notes | (2,049 | ) |  | (2,547 | )
Unamortized debt issuance costs - 2018 Notes | — |  |  | (209 | )
Total convertible notes | $ | 141,934 |  |  | $ | 213,898 |
Less current portion | — |  |  | 78,451 |
Total long-term convertible notes | $ | 141,934 |  |  | $ | 135,447 |
Schedule of interest expense on notes | Interest expense related to the notes for the years ended December 31, 2018, 2017 and 2016 was as follows:
| Years Ended December 31,
| 2018 |  | 2017 |  | 2016
| (in thousands)
2023 Notes coupon interest at a rate of 1.375% | $ | 2,372 |  |  | $ | 290 |  |  | $ | — |
2023 Notes amortization of discount and debt issuance cost at an additional effective interest rate of 4.9% | 6,486 |  |  | 768 |  |  | — |
2018 Notes coupon interest at a rate of 1.125% | 377 |  |  | 1,488 |  |  | 1,553 |
2018 Notes amortization of discount and debt issuance cost at an additional effective interest rate of 5.5% | 2,756 |  |  | 6,810 |  |  | 6,749 |
Total interest expense on convertible notes | $ | 11,991 |  |  | $ | 9,356 |  |  | $ | 8,302 |

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- Definition Tabular disclosure of information pertaining to short-term and long-debt instruments or arrangements, including but not limited to identification of terms, features, collateral requirements and other information necessary to a fair presentation. + References No definition available. + Details
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