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Stock based compensation (Narrative) (Details) (USD $)
In Millions, except Share data, unless otherwise specified
12 Months Ended
Dec. 31, 2013
Dec. 31, 2012
Dec. 31, 2011
Aug. 31, 2010
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]        
Number of shares authorized       18,500,000
Number of shares available for grant 9,800,000      
Stock based compensation expense $ 19.0 $ 8.2 $ 5.2  
Total compensation cost not yet recognized $ 34.5      
Total compensation cost not yet recognized, period for recognition 3 years      
Performance Shares [Member]
       
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]        
Granted (in shares) 110,720      
Employee performance period 3 years      
Performance Shares [Member] | Minimum [Member]
       
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]        
Number of shares for award settlement 0      
Performance Shares [Member] | Maximum [Member]
       
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]        
Number of shares for award settlement 2