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Share-based compensation plans (Details) (USD $)
In Millions, except Share data, unless otherwise specified
12 Months Ended
Dec. 31, 2011
years
Dec. 31, 2010
years
Dec. 31, 2009
years
Share-based compensation plans [Abstract]      
Number of shares underlying options 290,000 367,000 289,000
Weighted-average exercise price per share $ 31.47 $ 19.05 $ 27.59
Share-based Compensation Arrangement by Share-based Payment Award, Fair Value Assumptions and Methodology [Abstract]      
Expected dividend yield - weighted average 1.30% 2.10% 1.40%
Expected volatility - weighted average 36.00% 36.00% 36.00%
Expected volatility range - minimum 36.00% 35.00% 35.00%
Expected volatility range - maximum 37.00% 39.00% 39.00%
Risk-free interest rate - weighted average 1.20% 1.40% 1.80%
Risk-free interest rate range - minimum 0.50% 0.60% 0.90%
Risk-free interest rate range - maximum 1.90% 1.90% 2.40%
Expected term in years - weighted average 3.8 3.8 3.8
Expected term in years range - minimum 1.9 1.9 1.9
Expected term in years range - maximum 5.3 5.3 5.3
Share Based Compensation Arrangement By Share Based Payment Award Options Grants In Period Aggregate Weighted Average Grant Date Fair Value $ 2.4 $ 1.7 $ 2.1
Weighted-average fair value estimates at grant date $ 8.17 $ 4.65 $ 7.24
Nonvested share activity [Line Items]      
Beginning balance - shares 328,600 236,900 70,900
Stock units granted - shares 159,500 196,700 201,100
Cancelled awards - shares (16,500) (5,600) (1,300)
Vested - shares (156,200) (99,400) (33,800)
Ending balance - shares 315,400 328,600 236,900
Weighted-average grant-date fair value [Abstract]      
Beginning Balance - grant-date fair value $ 22.84 $ 28.45 $ 36.27
Stock units granted - grant-date fair value $ 30.43 $ 19.24 $ 26.90
Cancelled awards - grant-date fair value $ 23.65 $ 22.52 $ 26.80
Vested - grant-date fair value $ 24.13 $ 29.12 $ 35.71
Ending Balance - grant-date fair value $ 25.99 $ 22.84 $ 28.45
2005 Equity Incentive Plan
     
Nonvested share activity [Line Items]      
Beginning balance - shares 299,500 214,200 55,600
Stock units granted - shares 143,700 167,600 178,400
Cancelled awards - shares (16,500) (5,600) (1,300)
Vested - shares (127,100) (76,700) (18,500)
Ending balance - shares 299,600 299,500 214,200
Non-Employee Directors' Equity Plan
     
Nonvested share activity [Line Items]      
Beginning balance - shares 29,100 22,700 15,300
Stock units granted - shares 15,800 29,100 22,700
Cancelled awards - shares 0 0 0
Vested - shares (29,100) (22,700) (15,300)
Ending balance - shares 15,800 29,100 22,700