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Senior convertible notes (Tables)
9 Months Ended
Sep. 30, 2015
Summary of Senior Convertible Notes

Senior convertible notes consist of the following (in thousands):

 

     September 30,
2015
     December 31,
2014
 

2015 notes

     

Principal amount

   $ —         $ 100,000   

Unaccreted debt issuance costs

     —           (645
  

 

 

    

 

 

 

Net carrying amount

   $ —         $ 99,355   

2018 notes

     

Principal amount

   $ 27,690       $ —     

Unamortized premium

     717         —     

Unaccreted debt issuance costs

     (800   
  

 

 

    

 

 

 

Net carrying amount

   $ 27,607       $ —     
Accretion of Debt Issuance Expense

Accretion of debt issuance expense in connection with the 2015 and 2018 notes during the three and nine months ended September 30, 2015 and 2014 were as follows (in thousands):

 

     Three months ended
September 30,
     Nine months ended
September 30,
 
     2015      2014      2015      2014  

Accretion expense

   $ 195       $ 231       $ 673       $ 681