Share-Based Compensation (Narrative) (Details) (USD $) In Millions, except Share data, unless otherwise specified	1 Months Ended	12 Months Ended													3 Months Ended
	Jan. 31, 2013	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010	Dec. 31, 2012 Options [Member]	Dec. 31, 2011 Options [Member]	Dec. 31, 2010 Options [Member]	Dec. 31, 2012 Share Awards and Vesting [Member]	Dec. 31, 2011 Share Awards and Vesting [Member]	Dec. 31, 2010 Share Awards and Vesting [Member]	Dec. 31, 2012 Minimum [Member] Options [Member]	Dec. 31, 2011 Minimum [Member] Options [Member]	Dec. 31, 2012 Maximum [Member] Options [Member]	Dec. 31, 2011 Maximum [Member] Options [Member]	Jun. 30, 2011 2011 Share Incentive Plan [Member]	Dec. 31, 2012 2011 Share Incentive Plan [Member]
Total fungible units to be issued															9,100,000	7,900,000
Units available under the 2011 Share Plan															5,800,000
Fungible units transferred from the 2002 Share Plan to the 2011 Share Plan															3,300,000
Fungible Units To Full Value Award Conversion Ratio															3.45
Expiration value of option right or other award															0.83
Common shares which could be granted pursuant to full value awards																2,300,000
Exercise price range for options exercised during the period											$ 30.06	$ 30.06	$ 51.37	$ 62.32
Total intrinsic value of options exercised during the period					$ 12.2	$ 9.6	$ 1.5
Total unrecognized compensation cost related to unvested options which is expected to be amortized					0.4			33.1
Expected amortized period of unrecognized compensation expected to be recognized for share-based compensation plans					2 years
Remaining weighted average contractual life (years) outstanding options					3 years 329 days
Remaining weighted average contractual life (years) exercisable options					2 years 329 days
Vesting period, years											3 years		5 years
Options granted in current year		0	0	55,895	0
Fair value of shares vested								13.9	11.5	10.6
Total compensation cost for option and share awards		13.7	12.3	11.7
Total capitalized compensation cost for option and share awards		1.4	0.9	1.0
Percentage Of Market Value Per Share At Which Shares Purchased Under Employee Share Purchase Plan		85.00%
Employee Share Purchase Plan, Maximum Authorized Amount Per Participant		0
Shares purchased under the employee share purchase plan	4,985	20,137	19,914	29,100
Share Awards Held In Employee Trust Shares		1,900,000	2,000,000
Share Awards Held In Non Qualified Deferred Compensation Plan Shares		35.7	45.2
Cash Required By Plan Participants Upon Withdrawal		25.7	28.7
Share Awards Held In Non Qualified Deferred Compensation Plan Shares		1,000,000	900,000
Trading Securities Held In Non Qualified Deferred Compensation Plan Value		15.2	14.5
Defined Contribution Plan, Cost Recognized		$ 2.2	$ 1.8	$ 1.3