Long-term debt - Additional Required Disclosures (Details) - USD ($) $ in Thousands	12 Months Ended
Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
2027 Notes
Debt Instrument [Line Items]
Interest expense	$ 20,269	$ 15,569	$ 15,569
DFC Amortization	402	299	299
Amortization of Discount, net	133	0	0
2023 Notes
Debt Instrument [Line Items]
Interest expense	20,882	23,060	23,060
DFC Amortization	294	323	323
Amortization of Discount, net	$ 371	$ 402	$ 397

Amount of noncash expense included in interest expense to amortize debt discount and premium associated with the related debt instruments. Excludes amortization of financing costs. Alternate captions include noncash interest expense.

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Amount of amortization expense attributable to debt issuance costs.

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Amount of the cost of borrowed funds accounted for as interest expense for debt.

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