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Stock-based compensation - Summary of Company's Stock Appreciation Unit Activity (Details) - Stock Appreciation Units (SARs)
12 Months Ended
Dec. 31, 2018
$ / shares
shares
Number of Units  
Beginning Balance (in shares) | shares 239,824
Exercised/Converted (in shares) | shares (31,272)
Cancelled/Forfeited (in shares) | shares (15,680)
Ending Balance (in shares) | shares 192,872
Weighted-Average Exercise Price  
Beginning Balance (USD per share) | $ / shares $ 4.95
Exercised/Converted (USD per share) | $ / shares 4.32
Cancelled/Forfeited (USD per share) | $ / shares 6.72
Ending Balance (USD per share) | $ / shares $ 4.91