Employee Share-Based Incentive Compensation Plans (Details) (USD $)			12 Months Ended			1 Months Ended	12 Months Ended						1 Months Ended	12 Months Ended
	Dec. 31, 2011	Dec. 31, 2010	Dec. 31, 2011 OneBeacons employee stock ownership plan	Dec. 31, 2011 Sirius Group's profit sharing plan	Dec. 31, 2011 Performance shares Y multiplier	May 31, 2010 Non-Qualified Options	Dec. 31, 2011 Non-Qualified Options	Dec. 31, 2010 Non-Qualified Options	Dec. 31, 2009 Non-Qualified Options	Dec. 31, 2011 WTM Incentive Plan and WTM Phantom Share Plans Performance shares	Dec. 31, 2010 WTM Incentive Plan and WTM Phantom Share Plans Performance shares	Dec. 31, 2009 WTM Incentive Plan and WTM Phantom Share Plans Performance shares	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, 2010 OneBeacon Long-Term Incentive Plan Performance shares Performance cycle 2007-2009	Dec. 31, 2009 OneBeacon Long-Term Incentive Plan Performance shares Performance cycle 2007-2008	Dec. 31, 2011 OneBeacon Long-Term Incentive Plan Performance shares Performance cycle 2008-2010	Dec. 31, 2011 OneBeacon Long-Term Incentive Plan Non-Qualified Options	Dec. 31, 2010 OneBeacon Long-Term Incentive Plan Non-Qualified Options	Dec. 31, 2009 OneBeacon Long-Term Incentive Plan Non-Qualified Options
Share-based compensation arrangement by share-based payment award
Vesting period (in years)										3 years										Third, fourth and fifth anniversaries
Annual base contribution in common shares provided to participants expressed as a percentage of salary (as a percent)			3.00%
Social Security wage base amount, maximum			$ 106,800
Percentage of variable contribution for participants not otherwise eligible to receive certain other OneBeacon benefits			6.00%
Percentage of variable contribution for participants under Sirius Group's profit sharing plan				7.00%
Multiplier of number of shares initially granted to determine payout, low end of range					0
Multiplier of number of shares initially granted to determine payout, high end of range					2
Performance cycle (in years)					3
Target Performance Share activity
Beginning of period (in shares)										163,184	172,454	156,373		1,464,295	2,224,215	2,212,313
Shares expired (in shares)										(51,131)	(49,127)	(48,573)		(936,150)	(889,594)	(137,400)
New grants (in shares)										37,675	47,880	69,092		194,900	270,691	379,085
Cancellations and change in assumed forfeitures (in shares)										336	(8,023)	(4,438)	(35,000)	(80,378)	(141,017)	(229,783)
Ending of period (in shares)										150,064	163,184	172,454		642,667	1,464,295	2,224,215
Expense Amortized
Accrued expense at the beginning of period	187,900,000	157,400,000								29,400,000	12,200,000	4,200,000		18,500,000	15,100,000	4,600,000				4,500,000	3,600,000	2,500,000
Payments and deferrals														(10,500,000)	(4,600,000)
Assumed forfeitures and cancellations										(900,000)	(700,000)	(300,000)		(500,000)	(2,200,000)	(500,000)
Expense (income) recognized						8,700,000	100,000	11,900,000	5,400,000	37,600,000	17,900,000	8,300,000		2,200,000	10,200,000	11,000,000				500,000	900,000	1,100,000
Accrued expense at the end of the period	187,900,000	157,400,000								66,100,000	29,400,000	12,200,000		9,700,000	18,500,000	15,100,000				5,000,000	4,500,000	3,600,000
Percentage of target achieved by former employees as a result of the Commercial Lines and Personal Lines Transactions																	14.20%	1.40%	68.50%
Target vesting percentage of performance shares										100.00%				100.00%
Additional compensation cost that would be recognized if all outstanding performance shares vested										$ 19,300,000				$ 2,300,000