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Stock award plan - Restricted Stock (Details) - Unvested restricted stock
 3 Months Ended
Mar. 31, 2017
$ / shares
shares
Number of Shares  
Balance at the beginning of the period (in shares) | shares 271,651
Granted (in shares) | shares 328,194
Vested (in shares) | shares 30,161
Forfeited (in shares) | shares (3,090)
Balance at the end of the period (in shares) | shares 566,594
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 11.23
Vested (in dollars per share) | $ / shares 30.86
Forfeited (in dollars per share) | $ / shares 13.98
Balance at the end of the period (in dollars per share) | $ / shares $ 14.26