Quarterly report pursuant to Section 13 or 15(d)

Shareholders' Equity (Details)

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Shareholders' Equity (Details) (USD $)
Share data in Millions, except Per Share data
 1 Months Ended 3 Months Ended 6 Months Ended
May 31, 2011
Feb. 28, 2011
Jun. 30, 2011
Jun. 30, 2010
Jun. 30, 2011
Jun. 30, 2010
Jun. 07, 2011
Mar. 22, 2011
May 03, 2010
Apr. 30, 2009
Shareholders' Equity (Textuals)                    
Cash dividends per common share $ 0.20 $ 0.13                
Aggregate amount of cash dividend paid             $ 23,900,000 $ 14,800,000    
Dividends paid     (23,900,000) (12,000,000) (38,689,000) (24,061,000)        
Authorization of share repurchase program during next two years                   300,000,000
Renewed share repurchase authorization capacity                 1,000,000,000  
Previous expiration date of share repurchase program Apr. 01, 2011
Revised expiration date of share repurchase program Dec. 01, 2014
Repurchase of common shares through open market purchases, Share     1.8              
Share repurchased through open market purchases , Aggregate cost     98,800,000              
Average cost per share of share repurchased     $ 54.15              
Share repurchase program, Remaining authorized capacity     $ 677,900,000   $ 677,900,000          
Stock split         2 for 1