Stock award plan - Restricted Stock (Details) - Unvested restricted stock	9 Months Ended
	Sep. 30, 2017 $ / shares shares
Number of Shares
Balance at the beginning of the period (in shares) | shares	271,651
Granted (in shares) | shares	363,194
Vested (in shares) | shares	(180,861)
Forfeited (in shares) | shares	(51,131)
Balance at the end of the period (in shares) | shares	402,853
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	11.64
Vested (in dollars per share) | $ / shares	14.19
Forfeited (in dollars per share) | $ / shares	13.90
Balance at the end of the period (in dollars per share) | $ / shares	$ 15.62