Employee Share-Based Incentive Compensation Plans (WTM - Performance Shares) (Details) (USD $) In Millions, except Share data, unless otherwise specified			9 Months Ended	3 Months Ended				9 Months Ended
	Sep. 30, 2013	Dec. 31, 2012	Sep. 30, 2012 Performance Share Performance cycle 2009-2011	Sep. 30, 2013 Performance Share W T M Incentive and Phantom Plan [Member]		Sep. 30, 2012 Performance Share W T M Incentive and Phantom Plan [Member]		Sep. 30, 2013 Performance Share W T M Incentive and Phantom Plan [Member]		Sep. 30, 2012 Performance Share W T M Incentive and Phantom Plan [Member]		Sep. 30, 2013 Performance Share W T M Incentive and Phantom Plan [Member] Performance Cycle 2010 to 2012	Sep. 30, 2012 Performance Share W T M Incentive and Phantom Plan [Member] Performance cycle 2009-2011
Target Performance Share activity
Beginning of period (in shares)				118,976		118,450		119,357		150,064
Shares paid or expired (in shares)				0	[1]	0	[1]	(47,310)	[1]	(68,357)	[1]
New grants (in shares)				250		2,500		47,170		38,432
Assumed forfeitures and cancellations				(6)	[2]	(1,359)	[2]	3	[2]	(548)	[2]
Ending of period (in shares)				119,220		119,591		119,220		119,591
Accrued Expense
Accrued expense at the beginning of period	$ 161.6	$ 159.0		$ 34.9		$ 32.7		$ 29.4		$ 66.1
Payments and deferrals				0	[1]	0	[1]	(11.0)	[1]	(48.4)	[1]
Assumed forfeitures and cancellations				(0.2)	[2]	(0.4)	[2]	(0.2)	[2]	0.4	[2]
Expense recognized				7.1		6.6		23.6		20.8
Accrued expense at the end of the period	161.6	159.0		41.8		38.9		41.8		38.9
Vesting Period				3 years
Percentage of target payout - minimum actual												33.00%	147.00%
Percentage of target payout - maximum actual												98.00%	155.00%
Vested (in shares)			9,577
Additional compensation cost that would be recognized if all outstanding performance shares vested				$ 35.5				$ 35.5

[1] WTM performance share payments in 2013 for the 2010-2012 performance cycle ranged from 33% to 98% of target. WTM performance share payments in 2012 for the 2009-2011 performance cycle ranged from 147% to 155% of target.

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