Significant Transactions (Details) (USD $) In Millions, except Share data, unless otherwise specified	12 Months Ended		62 Months Ended
	Dec. 31, 2011	Dec. 31, 2010	Dec. 31, 2011
Significant Transactions
Common shares repurchased and retired during the period (in shares)	313,967	677,125	1,411,703
Value of common shares repurchased and retired during the period	$ 114.2	$ 222.0	$ 537.5
Tender Offer
Significant Transactions
Common shares repurchased and retired during the period (in shares)	332,346
Value of common shares repurchased and retired during the period	$ 138.8
Number of modified Dutch auction	2
Average price per share	$ 417.55