Employee Share-Based Incentive Compensation Plans (WTM - Restricted Shares) (Details) (USD $) In Millions, except Share data, unless otherwise specified	3 Months Ended
	Mar. 31, 2013	Mar. 31, 2012
Restricted Share Cliff Vesting in January 2016 [Member]
Target Performance Share activity
Issued (in shares)	25,470
Restricted Share Cliff Vesting in January 2015
Target Performance Share activity
Issued (in shares)		25,460
Restricted Share Vesting Beginning in February 2014
Target Performance Share activity
Issued (in shares)		3,000
Unamortized grant date fair value
Number of restricted shares installments		2
W T M Incentive Plan | Restricted Stock
Target Performance Share activity
Beginning of period (in shares)	69,910	72,000
Issued (in shares)	25,470	28,460
Vested (in shares)	0	(7,000)
Assumed forfeitures and cancellations	0	0
Ending of period (in shares)	95,380	93,460
Unamortized grant date fair value
Unamortized grant date fair value, beginning balance	$ 16.8	$ 13.3
Issued	14.3	13.7
Assumed forfeitures and cancellations	0	0
Expense recognized	(2.9)	(2.5)
Unamortized grant date fair value, ending balance	$ 28.2	$ 24.5