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Senior convertible notes (Tables)
6 Months Ended
Jun. 30, 2012
Senior convertible notes [Abstract]  
Summary of senior convertible notes
                 
    June 30
2012
    December 31
2011
 

Notes due 2013

               

Principal amount

  $ 115,000     $ 115,000  

Unaccreted debt issuance expense

    (852     (1,140
   

 

 

   

 

 

 

Net carrying amount

    114,148       113,860  
   

 

 

   

 

 

 

Notes due 2015

               

Principal amount

  $ 100,000     $ 100,000  

Unaccreted debt issuance expense

    (2,964     (3,218
   

 

 

   

 

 

 

Net carrying amount

    97, 036       96,782  
   

 

 

   

 

 

 

Senior convertible notes

  $ 211,184     $ 210,642  
   

 

 

   

 

 

 
Summary of amortization of debt issuance expense
                                 
    Three months ended
June  30,
    Six months ended
June  30,
 
    2012     2011     2012     2011  

Amortization expense

  $ 343     $ 324     $ 682     $ 644