Employee Share-Based Incentive Compensation Plans (Details 3) (USD $) In Millions, except Share data, unless otherwise specified	12 Months Ended			3 Months Ended		12 Months Ended			3 Months Ended		3 Months Ended	12 Months Ended
	Dec. 31, 2011	Dec. 31, 2010	Dec. 31, 2009	Jun. 30, 2011 WTM Restricted Stock	Mar. 31, 2011 WTM Restricted Stock	Dec. 31, 2011 WTM Restricted Stock	Dec. 31, 2010 WTM Restricted Stock	Dec. 31, 2009 WTM Restricted Stock	Mar. 31, 2007 WTM Restricted Stock Chairman and CEO	Dec. 31, 2010 WTM Restricted Stock Chairman and CEO installments	Mar. 31, 2010 Restricted shares that will vest in December, 2012	Dec. 31, 2009 Restricted shares cliff that will vest in November, 2010	Dec. 31, 2009 Restricted shares cliff that will vest in December, 2010	Dec. 31, 2009 Restricted shares that vest in three equal annual installments beginning in December 2011
Activity in plan
Beginning of period (in shares)					46,250	46,250	91,900	53,200
Issued (in shares)	27,250	19,750	47,820	250	27,000	27,250	19,750	47,100	15,000		19,750	2,500	40,100	4,500
Vested (in shares)						(1,500)	(65,150)	(8,400)
Cancellations and change in assumed forfeitures (in shares)							(250)
Ending of period (in shares)						72,000	46,250	91,900
Unamortized grant date fair value
Unamortized grant date fair value, beginning balance					$ 14.1	$ 14.1	$ 23.7	$ 24.2
Issued						9.9	6.7	9.6
Forfeited							(0.1)
Modified							(3.3)
Expense recognized						(10.7)	(12.9)	(10.1)
Unamortized grant date fair value, ending balance						$ 13.3	$ 14.1	$ 23.7
Number of installments in which the shares vest (in installments)										3