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Share-based compensation plans - Option Activity (Details) - Time Based Vesting Option
9 Months Ended
Sep. 30, 2025
$ / shares
shares
Share-based Compensation Arrangement by Share-based Payment Award, Options, Outstanding [Roll Forward]  
Beginning balance, shares | shares 22,500
Exercised, shares | shares (19,700)
Ending balance, shares | shares 2,800
Weighted Average Grant Date Fair Value Per Share  
Beginning balance (dollars per share) | $ / shares $ 20.98
Exercised (dollars per share) | $ / shares 21.15
Ending balance (dollars per share) | $ / shares $ 19.75