Employee Share-Based Incentive Compensation Plans (OneBeason - Peformance Shares) (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended		12 Months Ended
	Sep. 30, 2015	Sep. 30, 2014	Sep. 30, 2015	Sep. 30, 2014	Dec. 31, 2014
Expense Amortized
Accrued expense at the beginning of period			$ 108.1
Accrued expense at the end of the period	$ 121.0		$ 121.0		$ 108.1
OneBeacon | Performance Share | One Beacon Long Term Incentive Plan
Share-based compensation arrangement by share-based payment award
Vesting Period			3 years
OneBeacon | One Beacon Long Term Incentive Plan | Range Of Performance Cycle From 2012 To 2014 [Member]
Expense Amortized
Percentage of target payout - minimum actual			45.70%
OneBeacon | One Beacon Long Term Incentive Plan | Performance Cycle 2011 to 2013
Expense Amortized
Percentage of target payout - minimum actual			37.10%
OneBeacon | One Beacon Long Term Incentive Plan | Performance Share
Target Performance Shares Outstanding
Beginning of period (in shares)	462,147	512,938	517,470	493,421	493,421
Shares paid or expired (in shares)	0	0	(181,290)	(142,138)
New grants (in shares)	0	0	154,887	165,800
Assumed forfeitures and cancellations	0	0	(28,920)	(4,145)
Ending of period (in shares)	462,147	512,938	462,147	512,938	517,470
Expense Amortized
Accrued expense at the beginning of period	$ 2.4	$ 4.4	$ 3.4	$ 4.0	$ 4.0
Payments and deferrals	0.0	0.0	(1.5)	(1.0)
Assumed forfeitures and cancellations	0.0	0.0	0.0	0.0
Expense recognized	(0.3)	1.1	0.2	2.5
Accrued expense at the end of the period	2.1	$ 5.5	2.1	$ 5.5	$ 3.4
Additional compensation cost that would be recognized if all outstanding performance shares vested	$ 2.0		$ 2.0