Employee Share-Based Incentive Compensation Plans (WTM - Performance Shares Granted Under the WTM Incentive Plan) (Details) (USD $) In Millions, except Share data, unless otherwise specified	Sep. 30, 2013	Dec. 31, 2012
Share-based compensation arrangement by share-based payment award
Total Accrued Expense	$ 161.6	$ 159.0
Performance Share | W T M Incentive Plan [Member]
Share-based compensation arrangement by share-based payment award
Target Performance Shares Outstanding	122,277
Accrued Expense	42.9
Target Performance Shares Outstanding, Assumed forfeitures	(3,057)
Accrued Expense, Assumed forfeitures	(1.1)
Total Target Performance Shares Outstanding	119,220
Total Accrued Expense	41.8
Performance Share | W T M Incentive Plan [Member] | Performance Cycle 2013 to 2015
Share-based compensation arrangement by share-based payment award
Target Performance Shares Outstanding	47,170
Accrued Expense	8.0
Performance Share | W T M Incentive Plan [Member] | Performance Cycle 2012 To 2014
Share-based compensation arrangement by share-based payment award
Target Performance Shares Outstanding	37,977
Accrued Expense	15.8
Performance Share | W T M Incentive Plan [Member] | Performance Cycle 2011 to 2013
Share-based compensation arrangement by share-based payment award
Target Performance Shares Outstanding	37,130
Accrued Expense	$ 19.1