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Share-based compensation plans (Details) (USD $)
In Millions, except Share data, unless otherwise specified
12 Months Ended
Dec. 31, 2012
Dec. 31, 2011
Dec. 31, 2010
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]      
Number of shares underlying options 396,000 290,000 367,000
Weighted-average exercise price per share $ 22.55 $ 31.47 $ 19.05
Share-based Compensation Arrangement by Share-based Payment Award, Fair Value Assumptions and Methodology [Abstract]      
Expected dividend yield - weighted average 1.80% 1.30% 2.10%
Expected volatility - weighted average 40.00% 36.00% 36.00%
Expected volatility range - minimum 39.00% 36.00% 35.00%
Expected volatility range - maximum 40.00% 37.00% 39.00%
Risk-free interest rate - weighted average 0.60% 1.20% 1.40%
Risk-free interest rate range - minimum 0.40% 0.50% 0.60%
Risk-free interest rate range - maximum 0.90% 1.90% 1.90%
Share Based Compensation Arrangement By Share Based Payment Award Options Grants In Period Aggregate Weighted Average Grant Date Fair Value $ 2.5 $ 2.4 $ 1.7
Weighted-average fair value estimates at grant date $ 6.32 $ 8.17 $ 4.65
Nonvested share activity [Rollforward]      
Beginning balance - shares 315,400 328,600 236,900
Stock units granted - shares 344,000 159,500 196,700
Cancelled awards - shares (21,300) (16,500) (5,600)
Vested - shares (207,200) (156,200) (99,400)
Ending balance - shares 430,900 315,400 328,600
Weighted-average grant-date fair value [Rollforward]      
Beginning Balance - grant-date fair value $ 25.99 $ 22.84 $ 28.45
Stock units granted - grant-date fair value $ 22.21 $ 30.43 $ 19.24
Cancelled awards - grant-date fair value $ 24.53 $ 23.65 $ 22.52
Vested - grant-date fair value $ 25.68 $ 24.13 $ 29.12
Ending Balance - grant-date fair value $ 23.19 $ 25.99 $ 22.84
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 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 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
2005 Equity Incentive Plan
     
Nonvested share activity [Rollforward]      
Beginning balance - shares 299,600 299,500 214,200
Stock units granted - shares 321,000 143,700 167,600
Cancelled awards - shares (21,300) (16,500) (5,600)
Vested - shares (191,400) (127,100) (76,700)
Ending balance - shares 407,900 299,600 299,500
Non-Employee Directors' Equity Plan
     
Nonvested share activity [Rollforward]      
Beginning balance - shares 15,800 29,100 22,700
Stock units granted - shares 23,000 15,800 29,100
Cancelled awards - shares 0 0 0
Vested - shares (15,800) (29,100) (22,700)
Ending balance - shares 23,000 15,800 29,100