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Shareholders' Equity (Narrative) (Details)
In Thousands, except Share data, unless otherwise specified
 1 Months Ended 6 Months Ended 6 Months Ended 1 Months Ended 1 Months Ended
May 31, 2010
USD ($)
Feb. 28, 2010
USD ($)
Jun. 30, 2011
USD ($)
Jun. 30, 2010
USD ($)
Apr. 30, 2012
USD ($)
Jan. 31, 2012
USD ($)
Oct. 31, 2011
USD ($)
Aug. 31, 2011
USD ($)
Jul. 31, 2011
CHF
Jun. 30, 2011
CHF
May 31, 2011
USD ($)
Dec. 31, 2010
Jun. 30, 2011
Voting Common Stock [Member]
Jun. 30, 2011
Nonvoting Common Stock [Member]
Jun. 30, 2011
Allied World Bermuda [Member]
USD ($)
Aug. 31, 2010
The Chubb Corporation ("Chubb") [Member]
USD ($)
Nov. 30, 2010
The Goldman Sachs Group, Inc ("Goldman Sachs") [Member]
USD ($)
Aug. 31, 2010
The Goldman Sachs Group, Inc ("Goldman Sachs") [Member]
USD ($)
Nov. 05, 2010
The Goldman Sachs Group, Inc ("Goldman Sachs") [Member]
Feb. 28, 2011
American International Group, Inc. ("AIG") [Member]
USD ($)
Common shares related to warrants granted                             5,500,000 2,000,000 1,500,000     2,000,000
Percent increase in share capital     20.00%                                  
Voting common shares     37,945,043                 38,089,226 37,901,183 43,860            
Number of voting shares in commercial register     7,690,260                                  
Share capital registered in commercial register                   119,404                    
Number of conditional capital voting shares     7,200,000                                  
Common shares, outstanding     37,945,043                 38,089,226 37,901,183 43,860            
Common shares exercise price per share                             $ 34.20 $ 34.20 $ 34.20     $ 34.20
Repurchase of warrant     53,620                         32,819 37,197     53,620
Dividends payable per share         $ 0.375 $ 0.375 $ 0.375 $ 0.375     $ 0.375                  
Par value reduction based on the CHF exchange rate                 0.30                      
Dividends declared per share $ 0.20 $ 0.20                                    
Dividends payable 10,017 10,092                                    
Shares repurchased     969,163                           3,159,793 5,000,000    
Common shares value     $ 60,000 $ 49,089                         $ 185,448 $ 250,000    
Common share per share     $ 61.91                           $ 58.69 $ 50.00    
Percentage discount of weighted average trading price of common share                                     0.50%