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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, 2019
USD ($)
increment
shares
Dec. 31, 2018
USD ($)
Dec. 31, 2017
USD ($)
Share-based compensation          
Share-based compensation expense     $ 10.0 $ 7.8 $ 5.4
Income tax benefit     $ 1.4 1.1 1.9
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 of vested stock     $ 3.2 2.7 3.5
Income tax benefit     0.5 $ 0.4 1.1
Unrecognized compensation cost     $ 4.8    
Weighted-average period over which unrecognized compensation cost expected to be recognized     2 years 6 months    
LTIP linked to TRS          
Share-based compensation          
Fair value of vested stock         1.9
Income tax benefit         0.7
Unrecognized compensation cost     $ 1.4    
Weighted-average period over which unrecognized compensation cost expected to be recognized     1 year 6 months    
LTIP awards linked to other performance conditions          
Share-based compensation          
Fair value of vested stock         4.2
Income tax benefit         $ 1.6
Unrecognized compensation cost     $ 5.1    
Weighted-average period over which unrecognized compensation cost expected to be recognized     1 year 6 months    
Equity and Incentive Plan (EIP)          
Share-based compensation          
Additional shares available for issuance | shares   1,500,000      
Shares remaining available for future issuance | shares     3,200,000    
Shares that can be issued upon vesting of outstanding units and achievement of performance goals | shares     700,000    
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     310,263