Significant Transactions (Share Repurchases) (Details) (USD $) In Millions, except Share data, unless otherwise specified	3 Months Ended
	Mar. 31, 2013	Mar. 31, 2012
Significant Transactions
Additional authorized repurchase of common shares	545,496
Common shares repurchased and retired during the period	140,224	977,019
Value of common shares repurchased and retired during the period	$ 79.0	$ 485.8
Average price per share	$ 564	$ 497
Common Stock
Significant Transactions
Common shares repurchased and retired during the period	140,000
Stock repurchased and retired during period, pursuant of employee benefit Plans, shares	224
Combination One | Tender Offer
Significant Transactions
Common shares repurchased and retired during the period		157,629
Value of common shares repurchased and retired during the period		76.0
Average price per share		$ 482
Combination Two | Tender Offer
Significant Transactions
Common shares repurchased and retired during the period		816,829
Value of common shares repurchased and retired during the period		$ 408.6
Average price per share		$ 500
Combination Three
Significant Transactions
Common shares repurchased and retired during the period		2,561