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Convertible debt (Tables)
 9 Months Ended
Sep. 30, 2021
Debt Disclosure [Abstract]  
Schedule of Debt [Table Text Block] The following is a summary of the Company's convertible senior notes as of September 30, 2021 (in thousands):
Date of IssuanceUnpaid Principal BalanceNet Carrying AmountContractual Interest Rates
CurrentNoncurrent
1% Convertible Notes due in 2024 ("2024 Notes") May 2019 $143,750 $— $141,433 1%
2.25% Convertible Notes due in 2027 ("2027 Notes")September 2020$150,000 $— $146,481 2.25%
Convertible Debt [Table Text Block]
The Notes consist of the following (in thousands):
September 30, 2021December 31, 2020
Liability component:
Principal$293,750 $293,750 
Less: debt discount and issuance cost, net of amortization(5,836)(75,722)
Net carrying amount$287,914 $218,028 
Equity component(1)
$— $80,098 
(1)     Recorded within additional paid-in capital in the unaudited condensed consolidated balance sheet. As of December 31, 2020, it included $47.2 million and $32.9 million related to the 2027 and 2024 Notes, respectively, which was net of $1.3 million and $1.1 million issuance cost in equity, respectively.

The following table sets forth total interest expense recognized related to the Notes (in thousands):
Three Months Ended September 30,Nine Months Ended September 30,
2021202020212020
Coupon interest$1,203 $500 $3,609 $1,219 
Amortization of debt issuance costs373 174 1,119 488 
Amortization of debt discount— 1,824 — 4,938 
Total$1,576 $2,498 $4,728 $6,645