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		3 Months Ended		9 Months Ended				3 Months Ended	9 Months Ended	3 Months Ended
	Mar. 31, 2012 Restricted Share Cliff Vesting in January 2015 [Member]	Sep. 30, 2012 Restricted Share Cliff Vesting in January 2015 [Member]	Sep. 30, 2012 Restricted Share Cliff Vesting in July 2015 [Member]	Sep. 30, 2012 Restricted Stock [Member]	Sep. 30, 2011 Restricted Stock [Member]	Sep. 30, 2012 Restricted Stock [Member]	Sep. 30, 2011 Restricted Stock [Member]	Sep. 30, 2012 Restricted Share Vesting Beginning in February 2014 [Member]	Mar. 31, 2012 Restricted Share Vesting Beginning in February 2014 [Member] Installment	Mar. 31, 2011 Restricted Share Cliff Vesting in January 2014 [Member]	Sep. 30, 2012 Restricted Share Cliff Vesting in January 2014 [Member]	Mar. 31, 2010 Restricted Share Vested in December 2012 [Member]
Target Performance Share activity
Beginning of period (in shares)				93,460	73,500	72,000	46,250
Issued (in shares)	25,460	2,500	1,200	3,700	0	32,160	27,250	3,000		27,000	250	19,750
Vested (in shares)				0	0	(7,000)	0
Assumed forfeitures and cancellations				(1,065)	0	(1,065)	0
Ending of period (in shares)				96,095	73,500	96,095	73,500
Unamortized grant date fair value
Unamortized grant date fair value, beginning balance				$ 21.5	$ 18.9	$ 13.3	$ 14.1
Issued				2.0	0	15.7	9.9
Assumed forfeitures and cancelations				(0.2)	0	(0.2)	0
Expense recognized				(3.2)	(2.8)	(8.7)	(7.9)
Unamortized grant date fair value, ending balance				$ 20.1	$ 16.1	$ 20.1	$ 16.1
Number of restricted shares installments									2