Employee Share-Based Incentive Compensation Plans (Details) (OneBeacon's share-based compensation plans, USD $) In Millions, except Share data, unless otherwise specified	3 Months Ended	9 Months Ended		3 Months Ended		9 Months Ended					3 Months Ended	9 Months Ended			3 Months Ended		9 Months Ended					0 Months Ended
	Sep. 30, 2011	Sep. 30, 2012	Sep. 30, 2011	Sep. 30, 2012 Performance shares	Sep. 30, 2011 Performance shares	Sep. 30, 2012 Performance shares	Sep. 30, 2011 Performance shares	Sep. 30, 2012 Performance shares Performance cycle 2010-2012	Sep. 30, 2012 Performance shares Performance cycle 2011-2013	Sep. 30, 2012 Performance shares Performance cycle 2012-2014	Sep. 30, 2012 Performance shares Performance cycle 2009-2011	Sep. 30, 2012 Performance shares Performance cycle 2009-2011	Sep. 30, 2012 Performance shares Performance cycle 2008-2010	Sep. 30, 2012 Performance shares Sub-total	Sep. 30, 2012 Restricted Shares	Sep. 30, 2011 Restricted Shares	Sep. 30, 2012 Restricted Shares	Sep. 30, 2011 Restricted Shares	Jun. 30, 2012 Restricted Shares	Dec. 31, 2011 Restricted Shares	Jun. 30, 2011 Restricted Shares	May 25, 2011 Restricted Shares Performance cycle 2011-2013	Feb. 26, 2008 Restricted Stock Units (RSUs)
Activity in plan
Target performance shares outstanding, beginning of period				560,577	671,727	642,667	1,464,295	238,658	151,563	181,290				571,511
Payments and deferrals (in shares)						(258,901)	(936,150)
New awards (in shares)						181,290	194,900										300,000	630,000					116,270
Forfeitures and net change in assumed forfeitures (in shares)				(3,354)	(19,583)	(7,833)	(70,901)															(35,000)
Target performance shares outstanding, end of period				557,223	652,144	557,223	652,144	238,658	151,563	181,290				571,511
Expense amortized
Accrued expense at beginning of period				$ 3.1	$ 10.0	$ 9.7	$ 18.5		$ 0.4	$ 0.5				$ 0.9
Payments and deferrals						(7.8)	(10.5)
Forfeitures and net change in assumed forfeitures					0.1		(0.2)
Expense recognized	0.1	5.0	0.4	(2.2)	(0.5)	(1.0)	1.8								(0.8)	(0.4)	(1.9)	(0.5)
Accrued expense at end of period				0.9	9.6	0.9	9.6		0.4	0.5				0.9
Additional disclosures
Percentage of target achieved (as a percent)											138.60%	138.60%	68.50%
Target vesting percentage of performance shares (as a percent)		100.00%		100.00%		100.00%
Expense not yet recognized															10.4	8.1	10.4	8.1	11.2	7.7	8.5
Assumed forfeitures (in shares)						(14,288)
Additional compensation cost that would be recognized if all outstanding performance shares vested				$ 1.7		$ 1.7