Employee Share-Based Incentive Compensation Plans (WTM - Restricted Shares) (Details) (USD $) In Millions, except Share data, unless otherwise specified	3 Months Ended		9 Months Ended
	Sep. 30, 2013	Sep. 30, 2012	Sep. 30, 2013	Sep. 30, 2012
Restricted Share Cliff Vesting in January 2015 [Member]
Target Performance Share activity
Issued (in shares)		27,960
Restricted Share Cliff Vesting on July 2015 [Member]
Target Performance Share activity
Issued (in shares)				1,200
Restricted Share Vesting Beginning in February 2014 [Member]
Target Performance Share activity
Issued (in shares)				3,000
Unamortized grant date fair value
Number of restricted shares installments		2		2
Restricted Share Cliff Vesting in January 2016 [Member]
Target Performance Share activity
Issued (in shares)	25,720
W T M Incentive Plan [Member] | Restricted Stock
Target Performance Share activity
Beginning of period (in shares)	95,380	93,460	69,910	72,000
Issued (in shares)	250	3,700	25,720	32,160
Vested (in shares)	0	0	0	(7,000)
Assumed forfeitures and cancellations	0	(1,065)	0	(1,065)
Ending of period (in shares)	95,630	96,095	95,630	96,095
Unamortized grant date fair value
Unamortized grant date fair value, beginning balance	$ 24.5	$ 21.5	$ 16.8	$ 13.3
Issued	0.1	2.0	14.4	15.7
Assumed forfeitures and cancellations	0	(0.2)	0	(0.2)
Expense recognized	(3.8)	(3.2)	(10.4)	(8.7)
Unamortized grant date fair value, ending balance	$ 20.8	$ 20.1	$ 20.8	$ 20.1