Employee Share-Based Incentive Compensation Plans (OneBeacon - Performance Shares Outstanding) (Details) (USD $) In Millions, except Share data, unless otherwise specified	Sep. 30, 2014	Jun. 30, 2014	Dec. 31, 2013	Sep. 30, 2013	Jun. 30, 2013	Dec. 31, 2012
Share-based compensation arrangement by share-based payment award
Accrued incentive compensation	$ 187.5		$ 218.3
OneBeacon | Performance Share | One Beacon Long Term Incentive Plan
Share-based compensation arrangement by share-based payment award
Target Performance Shares Outstanding	526,090
Incentive Compensation Payable before Forfeitures	5.6
Target Performance Shares Outstanding, Assumed forfeitures	(13,152)
Accrued Expense, Assumed forfeitures	(0.1)
Total Target Performance Shares Outstanding	512,938	512,938	493,421	489,867	489,867	563,190
Accrued incentive compensation	5.5	4.4	4.0	2.5	2.4	1.2
Performance Cycle 2012 To 2014 | OneBeacon | Performance Share | One Beacon Long Term Incentive Plan
Share-based compensation arrangement by share-based payment award
Target Performance Shares Outstanding	181,290
Incentive Compensation Payable before Forfeitures	2.8
Performance Cycle 2014 to 2016 | OneBeacon | Performance Share | One Beacon Long Term Incentive Plan
Share-based compensation arrangement by share-based payment award
Target Performance Shares Outstanding	165,800
Incentive Compensation Payable before Forfeitures	0.6
Performance Cycle 2013 to 2015 | OneBeacon | Performance Share | One Beacon Long Term Incentive Plan
Share-based compensation arrangement by share-based payment award
Target Performance Shares Outstanding	179,000
Incentive Compensation Payable before Forfeitures	$ 2.2