Employee Share-Based Incentive Compensation Plans (Details 3) (OneBeacon's share-based compensation plans, USD $) In Millions, except Share data, unless otherwise specified	3 Months Ended	9 Months Ended		0 Months Ended	3 Months Ended		9 Months Ended				0 Months Ended	3 Months Ended		9 Months Ended
	Sep. 30, 2011	Sep. 30, 2012	Sep. 30, 2011	Feb. 26, 2008 Restricted Stock Units (RSUs) item	Sep. 30, 2012 Restricted Shares	Sep. 30, 2011 Restricted Shares	Sep. 30, 2012 Restricted Shares	Sep. 30, 2011 Restricted Shares	May 25, 2012 Restricted Shares	Mar. 01, 2012 Restricted Shares	May 25, 2011 Restricted Shares Performance cycle 2011-2013	Sep. 30, 2012 Performance shares	Sep. 30, 2011 Performance shares	Sep. 30, 2012 Performance shares	Sep. 30, 2011 Performance shares
Employee Share-Based Incentive Compensation Plans
Shares granted				116,270			300,000	630,000						181,290	194,900
Proportion of award vests on each of Nov 9, 2009, 2010 and 2011 (as a percent)				one-third
Minimum percentage of growth in book value per share for vesting each tranche of units (as a percent)				4.00%
Number of tranches				3
Restricted shares issued									630,000	300,000
Forfeitures (in shares)											35,000	3,354	19,583	7,833	70,901
Activity in plan
Beginning of period (in shares)					930,000	630,000	630,000
New awards (in shares)				116,270			300,000	630,000						181,290	194,900
End of period (in shares)					930,000	630,000	930,000	630,000
Unamortized grant date fair value
Unamortized grant date fair value, beginning of period					$ 11.2	$ 8.5	$ 7.7
Issued							4.6	8.6
Expense recognized	0.1	5.0	0.4		(0.8)	(0.4)	(1.9)	(0.5)				(2.2)	(0.5)	(1.0)	1.8
Unamortized grant date fair value, end of period					$ 10.4	$ 8.1	$ 10.4	$ 8.1
Total number of performance share grants to be reduced							35,000