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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, 2019
$ / shares
shares
Number of Units  
Beginning Balance (in shares) | shares 192,872
Exercised/Converted (in shares) | shares (11,252)
Canceled (in shares) | shares (18,149)
Ending Balance (in shares) | shares 163,471
Weighted-Average Exercise Price  
Beginning Balance (USD per share) | $ / shares $ 4.91
Exercised/Converted (USD per share) | $ / shares 3.52
Canceled (USD per share) | $ / shares 4.84
Ending Balance (USD per share) | $ / shares $ 5.01