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	Mar. 31, 2013	Dec. 31, 2012
Share-based compensation arrangement by share-based payment award
Total Accrued Expense	$ 100.8	$ 159.0
W T M Incentive Plan | Performance Share
Share-based compensation arrangement by share-based payment award
Target Performance Shares Outstanding	122,027
Accrued Expense	29.4
Target Performance Shares Outstanding, Assumed forfeitures	(3,051)
Accrued Expense, Assumed forfeitures	(0.8)
Total Target Performance Shares Outstanding	118,976
Total Accrued Expense	28.6
W T M Incentive Plan | Performance Share | Performance Cycle 2013 to 2015
Share-based compensation arrangement by share-based payment award
Target Performance Shares Outstanding	46,920
Accrued Expense	2.2
W T M Incentive Plan | Performance Share | Performance Cycle 2012 To 2014
Share-based compensation arrangement by share-based payment award
Target Performance Shares Outstanding	37,977
Accrued Expense	11.4
W T M Incentive Plan | Performance Share | Performance Cycle 2011 to 2013
Share-based compensation arrangement by share-based payment award
Target Performance Shares Outstanding	37,130
Accrued Expense	$ 15.8