Employee Share-Based Incentive Compensation Plans (OneBeacon - Non-Qualified Options) (Details) (USD $) In Millions, except Share data, unless otherwise specified	12 Months Ended			12 Months Ended			1 Months Ended		12 Months Ended
	Dec. 31, 2010	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2012 One Beacon Long Term Incentive Plan	Dec. 31, 2011 One Beacon Long Term Incentive Plan	Dec. 31, 2010 One Beacon Long Term Incentive Plan	May 31, 2010 Non-Qualified Options	Nov. 30, 2006 Non-Qualified Options One Beacon Long Term Incentive Plan	Dec. 31, 2012 Non-Qualified Options One Beacon Long Term Incentive Plan		Dec. 31, 2011 Non-Qualified Options One Beacon Long Term Incentive Plan		Dec. 31, 2010 Non-Qualified Options One Beacon Long Term Incentive Plan		Nov. 16, 2010 Non-Qualified Options One Beacon Long Term Incentive Plan	May 27, 2008 Non-Qualified Options One Beacon Long Term Incentive Plan	Nov. 16, 2010 One Beacon Segment Non-Qualified Options One Beacon Long Term Incentive Plan	Mar. 31, 2008 One Beacon Segment Non-Qualified Options One Beacon Long Term Incentive Plan
Share-based compensation arrangement by share-based payment award
Granted (in shares)								1,420,000
Expected volatility (as a percent)									30.00%
Risk-free rate (as a percent)									4.60%
Forfeiture (as a percent)									5.00%
Expected dividend yield (as a percent)									3.40%
Expected life (in years)									5 years 6 months
Initial exercise price (in dollars per share)									$ 30
Amended exercise price of option (in dollars per share)															$ 25.47	$ 27.97
One Beacon Special Dividend Per Share																	$ 2.50	$ 2.03
Target Performance Share activity
Unvested shares outstanding (in shares)									740,870		768,652		1,015,610
New awards									0		0		0
Forfeitures									0		(27,782)		(37,044)
Vested and expired									(740,870)	[1]	0	[1]	(209,914)	[1]
Exercised (in shares)	(2,400)								0		0		0
Unvested shares outstanding (in shares)									0		740,870		768,652
Unamortized grant date fair value
Accrued expense at the beginning of period		$ 159.0	$ 187.9	$ 9.7	$ 18.5	$ 15.1
Expense recognized				(0.8)	2.2	10.2	(8.7)		0		0.5		0.9
Accrued expense at the end of the period		$ 159.0	$ 187.9	$ 1.2	$ 9.7	$ 18.5

[1] During the year ended December 31, 2010, as a result of the Commercial Lines Transaction and Personal Lines Transaction, 209,914 options vested that were unexercised and expired. All remaining options expired in May 2012.