Significant Transactions (Share Repurchases) (Details) (USD $) In Millions, except Share data, unless otherwise specified	3 Months Ended	9 Months Ended		81 Months Ended				1 Months Ended	12 Months Ended
	Sep. 30, 2012	Sep. 30, 2012	Sep. 30, 2011	Sep. 30, 2012	May 25, 2012	Dec. 31, 2010	Nov. 17, 2006	Mar. 31, 2012 Tender Offer	Dec. 31, 2011 Tender Offer Auction
Significant Transactions
Common shares repurchased and retired during the period	50,000	217,801	265,768	1,629,504				816,829	332,346
Value of common shares repurchased and retired during the period	$ 26.4	$ 107.6	$ 93.2	$ 645.1				$ 408.6	$ 138.8
Average price per share								$ 500	$ 418
Number of modified Dutch auction									2
Authorized repurchase of common shares							1,000,000
Additional authorized repurchase of common shares					1,000,000	600,000
Shares remaining under share repurchase authorization	970,496	970,496		970,496