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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 ($)
Dec. 31, 2017
USD ($)
increment
shares
Dec. 31, 2016
USD ($)
Dec. 31, 2015
USD ($)
Share-based compensation          
Share-based compensation expense (in dollars)     $ 5.4 $ 4.8 $ 6.5
Income tax benefit     $ 1.9 1.6 2.3
Long-term incentive plan (LTIP)          
Share-based compensation          
Performance period     3 years    
Exception to forfeiture, minimum requisite service period     12 months    
Unrecognized compensation cost     $ 0.9    
Payout low end of range     0.00%    
Payout high end of range     200.00%    
Intrinsic value of shares exercised     $ 1.9    
Tax benefit realized for the deduction of exercises     $ 0.7    
LTIP linked to TRS          
Share-based compensation          
Weighted-average period over which unrecognized compensation cost expected to be recognized     2 years    
Restricted stock units          
Share-based compensation          
Fair value of vested stock     $ 3.5 2.8 3.7
Income tax benefit     1.1 0.9 1.1
Unrecognized compensation cost     $ 4.0    
Weighted-average period over which unrecognized compensation cost expected to be recognized     2 years 4 months 24 days    
LTIP awards linked to other performance conditions          
Share-based compensation          
Fair value of vested stock     $ 4.2 3.6 4.7
Income tax benefit     1.6 $ 1.4 $ 1.8
Unrecognized compensation cost     $ 2.9    
Weighted-average period over which unrecognized compensation cost expected to be recognized     2 years    
Equity and Incentive Plan (EIP)          
Share-based compensation          
Additional shares available for issuance | shares 1,500,000        
Shares remaining available for future issuance | shares     3,300,000    
Shares that can be issued upon vesting of outstanding units and achievement of performance goals | shares     400,000    
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     85,428    
Nonemployee Director Stock Plan | Stock Compensation Plan          
Share-based compensation          
Share-based compensation expense (in dollars)   $ 0.2