Annual report pursuant to Section 13 and 15(d)

Shareholders' Equity (Details)

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Shareholders' Equity (Details) (USD $)
Share data in Millions, except Per Share data, unless otherwise specified
 3 Months Ended 12 Months Ended
Jun. 30, 2011
Sep. 30, 2010
Jun. 30, 2010
Dec. 31, 2011
Dec. 31, 2010
Dec. 31, 2009
May 03, 2010
Apr. 30, 2009
Shareholders' Equity (Textual) [Abstract]                
Common shares, shares outstanding       115.8 117.8 120.5    
Preference shares authorized       7.5        
Preference shares Par Value       $ 0.002        
Stock split       2 for 1        
Cash dividends per common share $ 0.20 $ 0.13 $ 0.10          
Increase in Cash dividends per common share $ 0.07 $ 0.03            
Dividends paid       $ (85,489,000) $ (53,740,000) $ (48,721,000)    
Share repurchase program capacity authorization             1,000,000,000 300,000,000
Previous Expiration Date of Share Repurchase Program       Apr. 01, 2011        
Revised expiration date of share repurchase program       Dec. 01, 2014        
Share repurchase program, Remaining authorized capacity       477,900,000        
Repurchase of common shares through open market purchases, Share       5.5 5.7 4.0    
Share repurchased through open market purchases, Aggregate cost       $ 298,800,000 $ 150,100,000 $ 73,200,000    
Average cost per share of share repurchased       $ 54.27 $ 26.13 $ 18.30