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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 ($)
Dec. 31, 2016
USD ($)
increment
shares
Dec. 31, 2015
USD ($)
Dec. 31, 2014
USD ($)
Share-based compensation          
Share-based compensation expense (in dollars)     $ 4.8 $ 6.5 $ 9.3
Income tax benefit (in dollars)     $ 1.6 2.3 3.4
Restricted shares          
Share-based compensation          
Fair value of vested stock (in dollars)         0.1
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 (as a percent)     0.00%    
Payout high end of range (as a percent)     200.00%    
Restricted stock units          
Share-based compensation          
Fair value of vested stock (in dollars)     $ 2.8 3.7 4.1
Income tax benefit (in dollars)     0.9 1.1 1.2
Unrecognized compensation cost     $ 4.2    
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 (in dollars)     $ 3.6 4.7 1.9
Income tax benefit (in dollars)     $ 1.4 $ 1.8 $ 0.8
Equity and Incentive Plan (EIP)          
Share-based compensation          
Additional shares available for issuance | shares 1,500,000        
Shares remaining available for future issuance | shares     3,400,000    
Shares that can be issued upon vesting of outstanding units and achievement of performance goals | shares     300,000    
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    
Stock Option and Incentive Plan (SOIP) | Stock appreciation rights (SARs)          
Share-based compensation          
Percentage of award exercisable in installments     25.00%    
Period during which the award vests and becomes exercisable in installments     4 years    
Period award expires if not exercised     10 years    
Nonemployee Director Stock Plan          
Share-based compensation          
Shares remaining available for future issuance | shares     121,198    
Nonemployee Director Stock Plan | Stock Compensation Plan          
Share-based compensation          
Share-based compensation expense (in dollars)   $ 0.2