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Senior convertible notes (Tables)
 6 Months Ended
Jun. 30, 2013
Summary of Senior Convertible Notes

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

 

     June 30
2013		     December 31
2012		  

2013 notes

     

Principal amount

    $ 115,000      $ 115,000   

Unaccreted debt issuance expense

      (256 )      (557 ) 
  

 

 

    

 

 

  

Net carrying amount

      114,744        114,443   
  

 

 

    

 

 

  

2015 notes

     

Principal amount

    $ 100,000      $ 100,000   

Unaccreted debt issuance expense

      (1,996 )      (2,417 ) 
  

 

 

    

 

 

  

Net carrying amount

      98,004        97,583   
  

 

 

    

 

 

  

Senior convertible notes

    $ 212,748      $ 212,026   
  

 

 

    

 

 

  
Accretion of Debt Issuance Expense

Accretion of debt issuance expense in connection with the offerings of the 2015 notes and the 2013 notes during the three and six months ended June 30, 2013 and 2012 were as follows (in thousands).

 

     Three months ended
June 30,		      Six months ended
June 30,		  
     2013      2012      2013      2012  

Accretion expense

    $ 364       $ 343       $ 722       $ 682