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Stock-Based Compensation (Details) (USD $)
In Millions, except Share data, unless otherwise specified
3 Months Ended
Mar. 31, 2013
Mar. 31, 2012
Stock-based compensation    
Pre-tax compensation expense $ 8.0 $ 5.4
2008 Executive Incentive Program
   
Stock-based compensation    
Shares available for grant under the plan 3,050,636  
2005 Nonemployee Director Equity Plan
   
Stock-based compensation    
Shares available for grant under the plan 268,001  
Stock options
   
Stock-based compensation    
Pre-tax compensation expense 1.1 1.8
Stock options granted (in shares) 283,975  
Maximum term life 10 years  
Weighted-average assumptions used to estimate grant date fair value of stock options    
Expected dividend yield (as a percent) 1.20% 1.50%
Expected life in years 5 years 8 months 16 days 5 years 9 months 11 days
Expected volatility (as a percent) 32.20% 36.00%
Risk-free interest rate (as a percent) 1.00% 0.90%
Weighted average fair value (in dollars per share) $ 17.58 $ 14.57
Stock options | Awards 2008 and thereafter
   
Stock-based compensation    
Portion of stock options vesting on the first anniversary 0.3333  
Portion of stock options vesting on the second anniversary 0.3333  
Portion of stock options vesting on the third anniversary 0.3333  
Restricted stock awards
   
Stock-based compensation    
Awards granted (in shares) 71,255  
Performance share awards
   
Stock-based compensation    
Awards granted (in shares) 71,255  
Aggregate grant date fair value of stock options, restricted stock awards and performance share awards $ 16.8  
Restricted and Performance Share Award | 2008 Executive Incentive Program
   
Stock-based compensation    
Shares available for grant under the plan 686,985  
Restricted and Performance Share Award | 2005 Nonemployee Director Equity Plan
   
Stock-based compensation    
Shares available for grant under the plan 38,001