Common Shareholders' Equity (Details) (USD $) In Millions, except Share data, unless otherwise specified	12 Months Ended			62 Months Ended
	Dec. 31, 2011	Dec. 31, 2010	Dec. 31, 2009	Dec. 31, 2011	Aug. 26, 2010	Nov. 17, 2006
Common Shares Repurchased and Retired
Authorized repurchase of common shares (in shares)						1,000,000
Additional authorized repurchase of common shares (in shares)					600,000
Shares remaining under share repurchase authorization	188,297			188,297
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
Shares repurchased by the entity that were not subject to the Share Repurchase Plan (in shares)	189	10,186	1,113
Common Shares Issued
Common shares issued, total (in shares)	29,432	23,050	52,420
Shares issued in satisfaction of options exercised (in shares)	0	2,400	3,600
Restricted shares issued to key management personnel (in shares)	27,250	19,750	47,820
Shares issued to directors (in shares)	2,182	900	1,000
Dividends on Common Shares
Cash dividends declared and paid	8.0	8.8	8.9
Dividends declared and paid (in dollars per share)	$ 1.00	$ 1.00	$ 1.00
Tender Offer
Common Shares Repurchased and Retired
Number of modified Dutch auction	2
Common shares repurchased and retired during the period (in shares)	332,346
Value of each common share repurchased and retired during the period (in dollars per share)	$ 417.55
Value of common shares repurchased and retired during the period	$ 138.8