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Stock option plan (Details 2) - Restricted stock
 12 Months Ended
Dec. 31, 2015
$ / shares
shares
Number of Shares  
Balance at the beginning of the period (in shares) | shares 718,400
Vested (in shares) | shares (361,919)
Forfeited (in shares) | shares (12,146)
Balance at the end of the period (in shares) | shares 344,335
Weighted Average Grant Date Fair Value  
Balance at the beginning of the period (in dollars per share) | $ / shares $ 10.72
Vested (in dollars per share) | $ / shares 10.60
Forfeited (in dollars per share) | $ / shares 10.84
Balance at the end of the period (in dollars per share) | $ / shares $ 10.85