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Senior convertible notes (Details 1) (USD $)
In Thousands, unless otherwise specified
3 Months Ended 6 Months Ended
Jun. 30, 2012
Jun. 30, 2011
Jun. 30, 2012
Jun. 30, 2011
Summary of amortization of debt issuance expense        
Amortization expense $ 343 $ 324 $ 682 $ 644