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Share-based compensation plans (Details) (USD $)
In Thousands, unless otherwise specified
12 Months Ended
Dec. 31, 2011
Dec. 31, 2010
Dec. 31, 2009
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]      
Share-based compensation expense $ 5,733 $ 6,150 $ 6,663
Income tax benefit (1,930) (2,123) (2,375)
Stock incentive plan [Member]
     
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]      
Common stock reserved for issuance under stock incentive plan (in shares) 4,000 4,000 4,000
Common stock available for issuance under stock incentive plan (in shares) 1,476    
Stock incentive plan, full value awards factor 2.29 2.29 2.29
Compensation expense for unvested awards not yet recognized 5,383    
Weighted-average period over which expense for unvested awards will be recognized (in years) 1.7    
Stock Options [Member]
     
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]      
Share-based compensation expense 3,633 2,967 3,213
Restricted shares and restricted stock units [Member]
     
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]      
Share-based compensation expense 1,799 2,866 3,135
Employee stock purchase plan [Member]
     
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]      
Share-based compensation expense $ 301 $ 317 $ 315