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Share-based compensation - Narrative (Details) - USD ($)
3 Months Ended 6 Months Ended
Mar. 01, 2014
Jun. 30, 2023
Jun. 30, 2022
Jun. 30, 2023
Jun. 30, 2022
Share-based compensation          
Income tax benefit from compensation expense   $ 800,000 $ 800,000 $ 1,100,000 $ 1,100,000
Restricted stock units          
Share-based compensation          
Fair value of vested stock       3,700,000 4,000,000
Income tax benefit from compensation expense       800,000 600,000
Unrecognized share based compensation   6,500,000   $ 6,500,000  
Weighted average period for recognition of unrecognized compensation cost (in years)       2 years  
Long-term Incentive Plan          
Share-based compensation          
Payment award, low end of range       0.00%  
Payment award, high end of range       200.00%  
Measurement period for total return to shareholders (in years)       3 years  
Award performance period (in years)       3 years  
LTIP linked to TRS          
Share-based compensation          
Fair value of vested stock       $ 0 800,000
Income tax benefit from compensation expense         100,000
Unrecognized share based compensation   2,100,000   $ 2,100,000  
Weighted average period for recognition of unrecognized compensation cost (in years)       1 year 6 months  
LTIP awards linked to other performance conditions          
Share-based compensation          
Fair value of vested stock       $ 2,900,000 3,200,000
Income tax benefit from compensation expense       600,000 $ 400,000
Unrecognized share based compensation   $ 6,500,000   $ 6,500,000  
Weighted average period for recognition of unrecognized compensation cost (in years)       1 year 4 months 24 days  
Equity and Incentive Plan          
Share-based compensation          
Number of additional shares authorized (in shares) 1,500,000        
Shares available for future issuance (in shares)   2,700,000   2,700,000  
Number of share issuable upon vesting and achievement of performance goals (in shares)   700,000   700,000  
Nonemployee Director Stock Plan          
Share-based compensation          
Shares available for future grant (in shares)   168,177   168,177