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Share-based compensation - Additional Information (Details)
$ in Millions
1 Months Ended 12 Months Ended
Mar. 01, 2014
shares
Jul. 31, 2016
USD ($)
Dec. 31, 2018
USD ($)
increment
shares
Dec. 31, 2017
USD ($)
Dec. 31, 2016
USD ($)
Share-based compensation          
Share-based compensation expense     $ 7.8 $ 5.4 $ 4.8
Income tax benefit     $ 1.1 1.9 1.6
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     $ 2.7 3.5 2.8
Income tax benefit     0.4 1.1 0.9
Unrecognized compensation cost     $ 4.0    
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     $ 1.2    
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 3.6
Income tax benefit       $ 1.6 $ 1.4
Unrecognized compensation cost     $ 4.5    
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     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          
Shares remaining available for future issuance | shares     46,607    
Nonemployee Director Stock Plan | Stock Compensation Plan          
Share-based compensation          
Share-based compensation expense   $ 0.2