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Share-based compensation - The 2011 Director Plan (Details) - USD ($)
$ in Millions
12 Months Ended
Dec. 31, 2022
Dec. 31, 2021
Dec. 31, 2020
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]      
Income tax benefit $ 2.1 $ 1.4 $ 1.0
Common stock      
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]      
Shares granted (in shares) 35,720 29,816 36,100
Fair value $ 1.5 $ 1.3 $ 1.3
Income tax benefit $ 0.4 $ 0.3 $ 0.3