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Share-based compensation Additional Information (Details)
$ in Millions
12 Months Ended
Mar. 01, 2014
shares
Dec. 31, 2014
USD ($)
increment
shares
Dec. 31, 2013
USD ($)
shares
Dec. 31, 2012
USD ($)
shares
Share-based compensation        
Income tax benefit (in dollars)   $ 3.4 $ 2.8 $ 2.4
Equity and Incentive Plan (EIP)        
Share-based compensation        
Additional shares available for issuance | shares 1,500,000      
Shares remaining available for future issuance | shares   3,600,000    
Shares that can be issued upon vesting of outstanding units and achievement of performance goals | shares   900,000    
Stock Option and Incentive Plan (SOIP)        
Share-based compensation        
Outstanding shares | shares   17,000    
Period of time from grant date after which awards become unrestricted   4 years    
Nonemployee Director Stock Plan        
Share-based compensation        
Shares remaining available for future issuance | shares   169,290    
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%    
Stock appreciation rights (SARs)        
Share-based compensation        
Intrinsic value of equity instruments other than options, vested   $ 0.6    
Expired (in shares) | shares   0 0 4,000
Stock appreciation rights (SARs) | Stock Option and Incentive Plan (SOIP)        
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    
Restricted shares        
Share-based compensation        
Fair value of vested stock (in dollars)   $ 0.1 $ 0.1 $ 0.9
Income tax benefit (in dollars)       $ 0.2
Shares vested | shares   4,503 4,502 37,802
Restricted shares | Equity and Incentive Plan (EIP)        
Share-based compensation        
The number of equal annual increments for which the awards become unrestricted (in installments) | increment   4    
Restricted stock units        
Share-based compensation        
Fair value of vested stock (in dollars)   $ 4.1 $ 3.7 $ 0.7
Income tax benefit (in dollars)   1.2 $ 0.9 $ 0.2
Unrecognized compensation cost   $ 4.4    
Weighted-average period over which unrecognized compensation cost expected to be recognized   2 years 5 months 24 days    
Shares vested | shares   144,702 118,885 25,728
LTIP linked to TRS        
Share-based compensation        
Fair value of vested stock (in dollars)   $ 0.0 $ 2.2 $ 0.6
Income tax benefit (in dollars)   0.0 $ 0.9 $ 0.2
Unrecognized compensation cost   $ 2.2    
Weighted-average period over which unrecognized compensation cost expected to be recognized   1 year 6 months    
Shares vested | shares   70,189 87,753 35,397
Shares settled | shares   70,205    
Expired (in shares) | shares   17,548    
LTIP awards linked to other performance conditions        
Share-based compensation        
Fair value of vested stock (in dollars)   $ 1.9 $ 0.6  
Income tax benefit (in dollars)   $ 0.8 $ 0.2  
Shares vested | shares   65,089 18,280 0
2012-2014 LTIP linked to other performance conditions        
Share-based compensation        
Unrecognized compensation cost   $ 3.4    
Weighted-average period over which unrecognized compensation cost expected to be recognized   1 year 6 months