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Equity - Restricted Stock and performance-based stock units (Details) - Restricted stock and performance-based stock units - shares
3 Months Ended
Mar. 31, 2018
Mar. 31, 2017
Restricted stock and performance based stock units activity    
Outstanding at the beginning of the year (in shares) 244,181 210,573
Granted (in shares) 147,990 132,641
Vested (in shares) (61,733) (65,683)
Outstanding at the end of the year (in shares) 330,438 277,531