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Convertible debt (Tables)
 12 Months Ended
Dec. 31, 2018
Debt Instrument [Line Items]  
Convertible Debt [Table Text Block]
The Notes consist of the following (in thousands):
 
 
December 31, 2018
 
December 31, 2017
Liability component:
 
 
 
 
Principal
 
$
250,000

 
$
250,000

Less: debt discount, net of amortization
 
(24,810
)
 
(36,797
)
Net carrying amount
 
$
225,190

 
$
213,203

 
 
 
 
 
Equity component (1)
 
$
37,560

 
$
37,560

(1) Recorded within additional paid-in capital in the Consolidated Balance Sheet. As of December 31, 2018, it included $28.7 million and $8.8 million related to the 2019 Notes and the 2047 Notes, respectively, net of $1.2 million and $0.3 million issuance cost in equity, respectively.

The following table sets forth total interest expense recognized related to the Notes (in thousands):
 
 
Year Ended December 31,
 
 
2018
 
2017
2.0% coupon
 
$
5,000

 
$
3,991

Amortization of debt issuance costs
 
1,419

 
1,127

Amortization of debt discount
 
10,567

 
8,100

Total
 
$
16,986

 
$
13,218