Employee Share-Based Incentive Compensation Plans (WTM Performance Shares) (Details) (USD $) In Millions, except Share data, unless otherwise specified			3 Months Ended				9 Months Ended					9 Months Ended
	Sep. 30, 2012	Dec. 31, 2011	Sep. 30, 2012 WTM Incentive Plan and WTM Phantom Share Plans Performance Share [Member]		Sep. 30, 2011 WTM Incentive Plan and WTM Phantom Share Plans Performance Share [Member]		Sep. 30, 2012 WTM Incentive Plan and WTM Phantom Share Plans Performance Share [Member]		Sep. 30, 2011 WTM Incentive Plan and WTM Phantom Share Plans Performance Share [Member]		Mar. 31, 2012 WTM Incentive Plan and WTM Phantom Share Plans Performance Share [Member]	Sep. 30, 2012 Performance cycle 2009-2011 Performance Share [Member]	Sep. 30, 2012 Performance cycle 2009-2011 WTM Incentive Plan and WTM Phantom Share Plans Performance Share [Member]
Target Performance Share activity
Beginning of period (in shares)			118,450		150,064		150,064		163,184		119,591
Shares paid or expired (in shares)			0	[1]	0	[1]	(68,357)	[1]	(51,131)	[1]
New grants (in shares)			2,500		0		38,432		37,675
Assumed forfeitures and cancellations			1,359	[2]	0	[2]	548	[2]	(336)	[2]
Ending of period (in shares)			119,591		150,064		119,591		150,064		119,591
Accrued Expense
Accrued expense at the beginning of period	$ 144.7	$ 187.9	$ 32.7		$ 56.5		$ 66.1		$ 29.4
Payments and deferrals			0	[1]	0	[1]	(48.4)	[1]	0	[1]
Assumed forfeitures and cancelations			(0.4)	[2]	0	[2]	0.4	[2]	(0.6)	[2]
Expense recognized			6.6		(1.0)		20.8		26.7
Accrued expense at the end of the period	144.7	187.9	38.9		55.5		38.9		55.5
Percentage of payout for minimum growth target percentage													147.00%
Percentage of payout for maximum growth target percentage													155.00%
Shares issued during the period related to performance shares earned												9,577
Additional compensation cost that would be recognized if all outstanding performance shares vested			$ 26.6				$ 26.6

[1] (1) WTM performance share payments in 2012 for the 2009-2011 performance cycle ranged from147% to 155% of target. There were no payments made in 2011 for the 2008-2010 performance cycle; those performance shares did not meet the threshold performance goals and expired.

[2] (2) Amounts include changes in assumed forfeitures, as required under GAAP.