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Shareholders' equity (Details) (USD $)
3 Months Ended 6 Months Ended
Jun. 30, 2011
Mar. 31, 2011
Jun. 30, 2010
Jun. 30, 2010
Dec. 31, 2010
Disclosure Shareholders Equity Details          
Dividends declared per Common Share $ 0.35 $ 0.33 $ 0.33 $ 0.64  
Common Shares in treasury - shares 4,862,746       6,151,707