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Share-based compensation - Additional Information (Details)
$ in Millions
12 Months Ended
Jun. 26, 2019
shares
Mar. 01, 2014
shares
Dec. 31, 2020
USD ($)
increment
shares
Dec. 31, 2019
USD ($)
Dec. 31, 2018
USD ($)
Share-based compensation          
Income tax benefit     $ 1.0 $ 1.4 $ 1.1
Long-term incentive plan (LTIP)          
Share-based compensation          
Performance period     3 years    
Exception to forfeiture, minimum requisite service period     12 months    
Payout low end of range     0.00%    
Payout high end of range     200.00%    
Restricted stock units          
Share-based compensation          
Fair value     $ 4.2 3.2 2.7
Income tax benefit     0.7 $ 0.5 $ 0.4
Unrecognized compensation cost     $ 5.0    
Weighted-average period over which unrecognized compensation cost expected to be recognized     2 years 7 months 6 days    
LTIP linked to TRS          
Share-based compensation          
Fair value     $ 2.6    
Income tax benefit     0.4    
Unrecognized compensation cost     $ 1.2    
Weighted-average period over which unrecognized compensation cost expected to be recognized     1 year 4 months 24 days    
LTIP awards linked to other performance conditions          
Share-based compensation          
Fair value     $ 7.6    
Income tax benefit     1.2    
Unrecognized compensation cost     $ 4.2    
Weighted-average period over which unrecognized compensation cost expected to be recognized     1 year 4 months 24 days    
Equity and Incentive Plan (EIP)          
Share-based compensation          
Additional shares available for issuance | shares   1,500,000      
Shares remaining available for future issuance | shares     3,000,000.0    
Shares that can be issued upon vesting of outstanding units and achievement of performance goals | shares     600,000    
Equity and Incentive Plan (EIP) | Restricted shares          
Share-based compensation          
The number of equal annual increments for which the awards become unrestricted (in installments) | increment     4    
Nonemployee Director Stock Plan          
Share-based compensation          
Additional shares available for issuance | shares 300,000        
Shares remaining available for future issuance | shares     274,163