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Stock award plan - Restricted Stock (Details) - Unvested restricted stock
9 Months Ended
Sep. 30, 2018
$ / shares
shares
Number of Shares  
Balance at the beginning of the period (in shares) | shares 393,011
Granted (in shares) | shares 354,691
Vested (in shares) | shares (113,795)
Forfeited (in shares) | shares (53,560)
Balance at the end of the period (in shares) | shares 580,347
Weighted Average Grant Date Fair Value  
Balance at the beginning of the period (in dollars per share) | $ / shares $ 15.64
Granted (in dollars per share) | $ / shares 19.09
Vested (in dollars per share) | $ / shares 16.36
Forfeited (in dollars per share) | $ / shares 17.24
Balance at the end of the period (in dollars per share) | $ / shares $ 17.60