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Significant Transactions (Details) (USD $)
In Millions, except Share data, unless otherwise specified
 3 Months Ended 6 Months Ended 78 Months Ended 1 Months Ended 12 Months Ended
Jun. 30, 2012
Jun. 30, 2011
Jun. 30, 2012
Jun. 30, 2011
Jun. 30, 2012
May 25, 2012
Dec. 31, 2010
Nov. 17, 2006
Mar. 31, 2012
Tender Offer
Dec. 31, 2011
Tender Offer
Auction
Significant Transactions                    
Additional authorized repurchase of common shares           1,000,000 600,000      
Authorized repurchase of common shares               1,000,000    
Common shares repurchased and retired during the period 10,172 19,295 167,801 265,768 1,579,504       816,829 332,346
Value of common shares repurchased and retired during the period $ 5.2 $ 7.2 $ 81.2 $ 93.2 $ 619.0       $ 408.6 $ 138.8
Number of modified Dutch auction                   2
Shares remaining under share repurchase authorization 1,020,496   1,020,496   1,020,496          
Average price per share                 $ 500 $ 418