Employee Share-Based Incentive Compensation Plans (Details 2) (USD $) In Millions, except Share data, unless otherwise specified	12 Months Ended
	Dec. 31, 2011	Dec. 31, 2010	Dec. 31, 2009	Dec. 31, 2008
Share-based compensation arrangement by share-based payment award
Accrued incentive compensation	$ 187.9	$ 157.4
WTM Incentive Plan | Performance shares
Share-based compensation arrangement by share-based payment award
Target Performance Shares Outstanding (in shares)	140,975
Accrued Expense	60.3
Assumed forfeitures (in shares)	(3,524)
Assumed forfeitures	(1.6)
Target Performance Shares Outstanding	137,451
Accrued incentive compensation	58.7
WTM Incentive Plan | Performance cycle 2009-2011 | Performance shares
Share-based compensation arrangement by share-based payment award
Target Performance Shares Outstanding (in shares)	60,410
Accrued Expense	42.6
Growth in intrinsic business value per share, target (as a percent)	10.00%
Growth in intrinsic business value per share, maximum (as a percent)	17.00%
Percentage of payout for maximum growth target percentage	200.00%
Growth in intrinsic business value per share, minimum (as a percent)	3.00%
WTM Incentive Plan | Performance cycle 2010-2012 | Performance shares
Share-based compensation arrangement by share-based payment award
Target Performance Shares Outstanding (in shares)	42,890
Accrued Expense	10.9
Growth in intrinsic business value per share, target (as a percent)	11.00%
Growth in intrinsic business value per share, maximum (as a percent)	18.00%
Percentage of payout for maximum growth target percentage	200.00%
Growth in intrinsic business value per share, minimum (as a percent)	4.00%
WTM Incentive Plan | Performance cycle 2011-2013 | Performance shares
Share-based compensation arrangement by share-based payment award
Target Performance Shares Outstanding (in shares)	37,675
Accrued Expense	6.8
Growth in intrinsic business value per share, target (as a percent)	10.00%
Growth in intrinsic business value per share, maximum (as a percent)	17.00%
Percentage of payout for maximum growth target percentage	200.00%
Growth in intrinsic business value per share, minimum (as a percent)	3.00%
WTM Phantom Share Plans | Performance shares
Share-based compensation arrangement by share-based payment award
Target Performance Shares Outstanding (in shares)	12,937
Accrued Expense	7.5
Assumed forfeitures (in shares)	(324)
Assumed forfeitures	(0.1)
Target Performance Shares Outstanding	12,613
Accrued incentive compensation	7.4
WTM Phantom Share Plans | Performance cycle 2009-2011 | Performance shares
Share-based compensation arrangement by share-based payment award
Target Performance Shares Outstanding (in shares)	7,947
Accrued Expense	6.0
WTM Phantom Share Plans | Performance cycle 2010-2012 | Performance shares
Share-based compensation arrangement by share-based payment award
Target Performance Shares Outstanding (in shares)	4,990
Accrued Expense	1.5
WTM Phantom Share Plans | Performance cycle 2011-2013 | Performance shares
Share-based compensation arrangement by share-based payment award
Assumed forfeitures (in shares)	0
Assumed forfeitures	0
OneBeacon Long-Term Incentive Plan | Performance shares
Share-based compensation arrangement by share-based payment award
Target Performance Shares Outstanding (in shares)	652,562
Accrued Expense	9.8
Assumed forfeitures (in shares)	(9,895)
Assumed forfeitures	(0.1)
Target Performance Shares Outstanding	642,667	1,464,295	2,224,215	2,212,313
Accrued incentive compensation	9.7	18.5	15.1	4.6
Growth in intrinsic business value per share, target (as a percent)	11.00%	12.00%
Growth in intrinsic business value per share, maximum (as a percent)	18.00%	19.00%
Percentage of payout for maximum growth target percentage	200.00%	200.00%
Growth in intrinsic business value per share, minimum (as a percent)	4.00%	5.00%
OneBeacon Long-Term Incentive Plan | Performance cycle 2011-2013 | Performance shares
Share-based compensation arrangement by share-based payment award
Target Performance Shares Outstanding (in shares)	151,563
Accrued Expense	0.9
OneBeacon Long-Term Incentive Plan | Performance cycle 2009-2011 | Performance shares
Share-based compensation arrangement by share-based payment award
Target Performance Shares Outstanding (in shares)	256,751
Accrued Expense	7.7
OneBeacon Long-Term Incentive Plan | Performance cycle 2010-2012 | Performance shares
Share-based compensation arrangement by share-based payment award
Target Performance Shares Outstanding (in shares)	244,248
Accrued Expense	$ 1.2