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Stock award plan - Restricted Stock (Details) - Unvested restricted stock
 6 Months Ended
Jun. 30, 2017
$ / shares
shares
Number of Shares  
Balance at the beginning of the period (in shares) | shares 271,651
Granted (in shares) | shares 358,194
Vested (in shares) | shares (180,861)
Forfeited (in shares) | shares (24,998)
Balance at the end of the period (in shares) | shares 423,986
Weighted Average Grant Date Fair Value  
Balance at the beginning of the period (in dollars per share) | $ / shares $ 19.76
Granted (in dollars per share) | $ / shares 11.34
Vested (in dollars per share) | $ / shares 14.19
Forfeited (in dollars per share) | $ / shares 13.47
Balance at the end of the period (in dollars per share) | $ / shares $ 15.39