Employee Share-Based Incentive Compensation Plans (OneBeacon - Performance Shares Outstanding) (Details) (USD $) In Millions, except Share data, unless otherwise specified	Jun. 30, 2013	Mar. 31, 2013	Dec. 31, 2012	Jun. 30, 2012	Mar. 31, 2012	Dec. 31, 2011
Share-based compensation arrangement by share-based payment award
Accrued incentive compensation	$ 125.9		$ 159.0
Performance Share | One Beacon Long Term Incentive Plan
Share-based compensation arrangement by share-based payment award
Target Performance Shares Outstanding	502,428
Incentive Compensation Payable before Forfeitures	2.5
Target Performance Shares Outstanding, Assumed forfeitures	(12,561)
Accrued Expense, Assumed forfeitures	(0.1)
Total Target Performance Shares Outstanding	489,867	489,867	563,190	560,577	569,745	642,667
Accrued incentive compensation	2.4	1.6	1.2	3.1	3.2	9.7
Performance Share | One Beacon Long Term Incentive Plan | Performance Cycle 2013 to 2015
Share-based compensation arrangement by share-based payment award
Target Performance Shares Outstanding	179,000
Incentive Compensation Payable before Forfeitures	0.5
Performance Share | One Beacon Long Term Incentive Plan | Performance Cycle 2012 To 2014
Share-based compensation arrangement by share-based payment award
Target Performance Shares Outstanding	181,290
Incentive Compensation Payable before Forfeitures	1.4
Performance Share | One Beacon Long Term Incentive Plan | Performance Cycle 2011 to 2013
Share-based compensation arrangement by share-based payment award
Target Performance Shares Outstanding	142,138
Incentive Compensation Payable before Forfeitures	$ 0.6