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Stock based compensation (Fair Value Assumptions) (Details) (USD $)
In Millions, except Per Share data, unless otherwise specified
12 Months Ended
Dec. 31, 2013
Dec. 31, 2012
Dec. 31, 2011
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]      
Intrinsic value of options exercised (less than $0.1 million in 2011) $ 19.2 $ 25.0 $ 0.1
Stock Options [Member]
     
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]      
Weighted average fair value (in dollars per share) $ 8.41 $ 6.81 $ 5.08
Expected life (in years) 6 years 3 months 6 years 3 months 6 years 3 months
Volatility 30.00% 36.00% 34.00%
Dividend yield 0.00% 0.00% 0.00%
Risk free interest rate 1.17%    
Risk free interest rate, minimum   1.13% 1.19%
Risk free interest rate, maximum   1.22% 2.64%