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Debt (Tables)
 12 Months Ended
Dec. 31, 2017
Debt Disclosure [Abstract]  
Summary of convertible notes
The Convertible Notes consist of the following:
 
 
Year ended
December 31,
Liability component
 
2017
 
2016
Principal
 
$
150,000

 
$
150,000

Less: Debt issuance costs
 
(2,121
)
 
(2,457
)
Less: Debt discount, net (1)
 
(42,572
)
 
(49,327
)
Net carrying amount
 
$
105,307

 
$
98,216

 
(1)
Included in the consolidated balance sheets within convertible senior notes (due 2022) and amortized to interest expense over the remaining life of the Convertible Notes using the effective interest rate method.
Summary of interest expense recognized related to the Convertible Notes
The following table sets forth total interest expense recognized related to the Convertible Notes:
 
 
Year ended
December 31,
 
 
2017
 
2016
Contractual interest expense
 
$
4,500

 
$
4,497

Amortization of debt issuance costs
 
337

 
302

Amortization of debt discount
 
6,755

 
6,065

Total
 
$
11,592

 
$
10,864

Effective interest rate of the liability component
 
11.0
%
 
11.0
%