Employee Share-Based Incentive Compensation Plans (Details 6) (USD $) In Millions, except Share data, unless otherwise specified	12 Months Ended			3 Months Ended	12 Months Ended				1 Months Ended		1 Months Ended	12 Months Ended
	Dec. 31, 2011 Qualified Plan	Dec. 31, 2010 Qualified Plan	Dec. 31, 2009 Qualified Plan	Mar. 31, 2008 OneBeacon Long-Term Incentive Plan Restricted Stock Units, RSU	Dec. 31, 2011 OneBeacon Long-Term Incentive Plan Restricted Stock Units, RSU	Dec. 31, 2010 OneBeacon Long-Term Incentive Plan Restricted Stock Units, RSU	Dec. 31, 2009 OneBeacon Long-Term Incentive Plan Restricted Stock Units, RSU	Dec. 31, 2011 OneBeacon Long-Term Incentive Plan OneBeacon Restricted Stock	May 31, 2011 OneBeacon Long-Term Incentive Plan OneBeacon Restricted Stock Chairman and CEO Y	May 25, 2011 OneBeacon Long-Term Incentive Plan OneBeacon Restricted Stock Chairman and CEO	May 31, 2011 OneBeacon Long-Term Incentive Plan Performance shares	Dec. 31, 2011 OneBeacon Long-Term Incentive Plan Performance shares	Dec. 31, 2010 OneBeacon Long-Term Incentive Plan Performance shares	Dec. 31, 2009 OneBeacon Long-Term Incentive Plan Performance shares	Dec. 31, 2008 OneBeacon Long-Term Incentive Plan Performance shares
Share-based compensation arrangement by share-based payment award
New grants (in shares)				116,270								194,900	270,691	379,085
Proportion of award vests on each of Nov 9, 2009, 2010 and 2011				0.33
Minimum percentage of growth in book value per share for vesting each tranche of units				4.00%
Expense recognized					$ 0.1	$ 0.5	$ 0.6					$ 2.2	$ 10.2	$ 11.0
Outstanding (in shares)												642,667	1,464,295	2,224,215	2,212,313
One Beacon Restricted shares issued (in shares)										630,000
Numerator value of the ratio in which restricted shares vest									1
Denominator value of the ratio in which restricted shares vest									4
Forfeiture of performance shares granted for the 2011 - 2013 performance cycle (in shares)											35,000	80,378	141,017	229,783
Period in which performance share awards will be reduced by a similar number of shares (in years)									5
Noncontrolling interest expense recognized for the dilution of ownership								7.7
Variable contribution amounts earned by eligible participants of the KSOP as a percentage of salary	2.00%	0.00%	4.00%
Compensation expense to pay benefits and allocate common shares	$ 6.3	$ 3.0	$ 11.0
Total common shares outstanding owned by the plans (as a percent)	3.00%	3.00%