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Share-based compensation - Narrative (Details) - USD ($)
$ in Millions
12 Months Ended
Mar. 01, 2014
Dec. 31, 2023
Dec. 31, 2022
Dec. 31, 2021
Dec. 31, 2020
Share-based compensation          
Income tax benefit   $ 2.2 $ 2.1 $ 1.4  
Long-term incentive plan (LTIP)          
Share-based compensation          
Performance period (in years)   3 years      
Exception to forfeiture, minimum requisite service period (in months)   12 months      
Payout low end of range (as percent)   0.00%      
Payout high end of range (as percent)   200.00%      
Restricted stock units          
Share-based compensation          
Fair value   $ 3.7 4.2 3.0  
Income tax benefit   0.8 0.6 0.6  
Unrecognized compensation cost   $ 4.1      
Weighted-average period over which unrecognized compensation cost expected to be recognized (in years)   1 year 9 months 18 days      
LTIP linked to TRS          
Share-based compensation          
Fair value   $ 0.0 0.8 0.8  
Income tax benefit     0.1 0.2  
Unrecognized compensation cost   $ 1.3      
Weighted-average period over which unrecognized compensation cost expected to be recognized (in years)   1 year 6 months      
LTIP awards linked to other performance conditions          
Share-based compensation          
Fair value   $ 2.9 3.2 1.7  
Income tax benefit   0.6 $ 0.4 $ 0.4  
Unrecognized compensation cost   $ 4.4      
Weighted-average period over which unrecognized compensation cost expected to be recognized (in years)   1 year 6 months      
Equity and Incentive Plan (EIP)          
Share-based compensation          
Additional shares available for issuance (in shares) 1,500,000        
Shares remaining available for future issuance (in shares)   2,700,000      
Shares that can be issued upon vesting of outstanding units and achievement of performance goals (in shares)   500,000      
Equity and Incentive Plan (EIP) | Restricted shares          
Share-based compensation          
Vesting period   3 years 3 years 3 years 4 years
Equity and Incentive Plan (EIP) | Restricted shares | Share-based Payment Arrangement, Tranche One          
Share-based compensation          
Vesting percentage   33.33% 33.33% 25.00% 25.00%
Equity and Incentive Plan (EIP) | Restricted shares | Share-based Payment Arrangement, Tranche Two          
Share-based compensation          
Vesting percentage   33.33% 33.33% 25.00% 25.00%
Equity and Incentive Plan (EIP) | Restricted shares | Share-based Payment Arrangement, Tranche Three          
Share-based compensation          
Vesting percentage   33.33% 33.33% 25.00% 25.00%
Equity and Incentive Plan (EIP) | Restricted shares | Share-Based Payment Arrangement, Tranche Four          
Share-based compensation          
Vesting percentage       25.00% 25.00%
Nonemployee Director Stock Plan          
Share-based compensation          
Shares remaining available for future issuance (in shares)   168,177