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Stock-based compensation - Narrative (Details) - USD ($)
$ in Thousands
3 Months Ended 6 Months Ended 12 Months Ended
Jun. 30, 2021
Jun. 30, 2020
Jun. 30, 2021
Jun. 30, 2020
Dec. 31, 2020
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]          
Compensation cost related to nonvested awards     $ 5,181 $ 4,233  
Dividends declared not paid on restricted stock units $ 128 $ 77 $ 128 77  
ESPP          
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]          
Number of shares reserved for issuance (in shares) 2,357,440   2,357,440   2,379,006
RSUs          
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]          
Fair value of restricted stock units vested and released $ 828 1,048 $ 5,192 4,299  
Compensation cost related to nonvested awards 2,193 2,015 4,659 3,817  
Unrecognized compensation cost related to nonvested awards 16,091   $ 16,091   $ 13,436
Expected weighted-average period to be recognized     2 years 8 months 12 days   2 years 6 months
Dividends declared not paid on restricted stock units 741   $ 741   $ 613
RSUs | Independent Directors          
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]          
Compensation cost related to nonvested awards $ 142 231 $ 299 378  
RSUs | 2016-LTIP Plan          
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]          
Number of shares available for issuable (in shares) 2,158,408   2,158,408   2,240,434
PSUs          
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]          
Compensation cost related to nonvested awards $ 322 $ 335 $ 522 $ 416  
Unrecognized compensation cost related to nonvested awards $ 7,829   $ 7,829    
Expected weighted-average period to be recognized     2 years 4 months 24 days    
Core return threshold percentages     200.00%    
Award vesting, percentage 140.00%   102.50%    
PSUs | Executives and Other Officers          
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]          
Criteria period     3 years    
Vesting period     3 years    
PSUs | Executives and Other Officers | Minimum          
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]          
Core return threshold percentages     0.00%    
PSUs | Executives and Other Officers | Maximum          
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]          
Core return threshold percentages     200.00%    
Employee Stock | ESPP          
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]          
Number of shares available for issuable (in shares) 200,000   200,000    
Purchase price percentage of subsequent offering periods     95.00%    
Maximum number of shares per participant (in shares)     725    
Maximum worth of award per participant     $ 25    
Shares issued under plan (in shares) 0 0 21,566,000 12,145,000