Employee Share-Based Incentive Compensation Plans (OneBeason - Peformance Shares) (Details) (USD $) In Millions, except Share data, unless otherwise specified			9 Months Ended	3 Months Ended		9 Months Ended
	Sep. 30, 2014	Dec. 31, 2013	Sep. 30, 2014 OneBeacon Performance Share One Beacon Long Term Incentive Plan	Sep. 30, 2014 OneBeacon One Beacon Long Term Incentive Plan Performance Share	Sep. 30, 2013 OneBeacon One Beacon Long Term Incentive Plan Performance Share	Sep. 30, 2014 OneBeacon One Beacon Long Term Incentive Plan Performance Share	Sep. 30, 2013 OneBeacon One Beacon Long Term Incentive Plan Performance Share	Sep. 30, 2014 Performance Cycle 2011 to 2013 OneBeacon One Beacon Long Term Incentive Plan
Share-based compensation arrangement by share-based payment award
Vesting Period			3 years
Target Performance Shares Outstanding
Beginning of period (in shares)				512,938	489,867	493,421	563,190
Shares paid or expired (in shares)				0	0	(142,138)	(238,658)
New grants (in shares)				0	0	165,800	179,000
Assumed forfeitures and cancellations				0	0	(4,145)	(13,665)
Ending of period (in shares)				512,938	489,867	512,938	489,867
Expense Amortized
Accrued expense at the beginning of period	$ 187.5	$ 218.3		$ 4.4	$ 2.4	$ 4.0	$ 1.2
Payments and deferrals				0	0	(1.0)	0
Assumed forfeitures and cancellations				0	0	0	(0.1)
Expense recognized				1.1	0.1	2.5	1.4
Accrued expense at the end of the period	187.5	218.3		5.5	2.5	5.5	2.5
Percentage of target payout - minimum actual								37.10%
Additional compensation cost that would be recognized if all outstanding performance shares vested				$ 3.7		$ 3.7