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Share-based compensation plans (Details) (USD $)
In Millions, except Share data, unless otherwise specified
3 Months Ended 9 Months Ended
Sep. 30, 2012
Sep. 30, 2011
Sep. 30, 2012
Sep. 30, 2011
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]        
Number of shares underlying options 182,000 290,000 389,000 290,000
Weighted-average exercise price per share $ 22.56 $ 31.47 $ 22.47 $ 31.47
Share-based Compensation Arrangement by Share-based Payment Award, Fair Value Assumptions and Methodology [Abstract]        
Expected dividend yield - weighted average 1.80% 1.30% 1.80% 1.30%
Expected dividend yield - range  1.8  1.3  1.8  1.3
Expected volatility - weighted average 40.00% 36.00% 40.00% 36.00%
Expected volatility range - minimum   36.00%   36.00%
Expected volatility range - maximum 40.00% 37.00% 40.00% 37.00%
Risk-free interest rate - weighted average 0.60% 1.20% 0.70% 1.20%
Risk-free interest rate range - minimum 0.40% 0.50% 0.40% 0.50%
Risk-free interest rate range - maximum 0.80% 1.90% 0.90% 1.90%
Share Based Compensation Arrangement By Share Based Payment Award Options Grants In Period Aggregate Weighted Average Grant Date Fair Value $ 1.2 $ 2.4 $ 2.5 $ 2.4
Weighted-average fair value estimates at grant date $ 6.31 $ 8.17 $ 6.30 $ 8.17
Nonvested share activity [Rollforward]        
Beginning balance - shares     315,400  
Stock units granted - shares     338,100  
Cancelled awards - shares     (13,500)  
Vested - shares     (200,700)  
Ending balance - shares 439,300   439,300  
Weighted Average [Memeber]
       
Share-based Compensation Arrangement by Share-based Payment Award, Fair Value Assumptions and Methodology [Abstract]        
Expected term in years 4 years 3 months 0 days 3 years 9 months 11 days 4 years 3 months 0 days 3 years 9 months 11 days
Minimum [Member]
       
Share-based Compensation Arrangement by Share-based Payment Award, Fair Value Assumptions and Methodology [Abstract]        
Expected term in years 3 years 3 months 0 days 1 year 10 months 15 days 3 years 3 months 0 days 1 year 10 months 15 days
Maximum [Member]
       
Share-based Compensation Arrangement by Share-based Payment Award, Fair Value Assumptions and Methodology [Abstract]        
Expected term in years 5 years 3 months 0 days 5 years 3 months 0 days 5 years 3 months 0 days 5 years 3 months 0 days
2005 Equity Incentive Plan
       
Nonvested share activity [Rollforward]        
Beginning balance - shares     299,600  
Stock units granted - shares     315,100  
Cancelled awards - shares     (13,500)  
Vested - shares     (184,900)  
Ending balance - shares 416,300   416,300  
Weighted-average grant-date fair value [Rollforward]        
Beginning Balance - grant-date fair value     $ 25.99  
Stock units granted - grant-date fair value     $ 22.13  
Cancelled awards - grant-date fair value     $ 24.85  
Vested - grant-date fair value     $ 25.69  
Ending Balance - grant-date fair value $ 23.20   $ 23.20  
Non-Employee Directors' Equity Plan
       
Nonvested share activity [Rollforward]        
Beginning balance - shares     15,800  
Stock units granted - shares     23,000  
Cancelled awards - shares     0  
Vested - shares     (15,800)  
Ending balance - shares 23,000   23,000  
Weighted-average grant-date fair value [Rollforward]        
Beginning Balance - grant-date fair value     $ 25.99  
Stock units granted - grant-date fair value     $ 22.13  
Cancelled awards - grant-date fair value     $ 24.85  
Vested - grant-date fair value     $ 25.69  
Ending Balance - grant-date fair value $ 23.20   $ 23.20