Employee Share-Based Incentive Compensation Plans (Details 5) (USD $) In Millions, except Share data, unless otherwise specified			1 Months Ended	3 Months Ended		6 Months Ended	9 Months Ended					3 Months Ended	9 Months Ended
	Sep. 30, 2011	Dec. 31, 2010	Nov. 30, 2006 OneBeacon Long-Term Incentive Plan Non-Qualified Options	Sep. 30, 2011 OneBeacon Long-Term Incentive Plan Non-Qualified Options	Sep. 30, 2010 OneBeacon Long-Term Incentive Plan Non-Qualified Options	Jun. 30, 2010 OneBeacon Long-Term Incentive Plan Non-Qualified Options	Sep. 30, 2011 OneBeacon Long-Term Incentive Plan Non-Qualified Options year	Sep. 30, 2010 OneBeacon Long-Term Incentive Plan Non-Qualified Options	Nov. 16, 2010 OneBeacon Long-Term Incentive Plan Non-Qualified Options	May 27, 2008 OneBeacon Long-Term Incentive Plan Non-Qualified Options	Mar. 31, 2008 OneBeacon Long-Term Incentive Plan Non-Qualified Options	Sep. 30, 2010 Non-Qualified Options	Sep. 30, 2011 Non-Qualified Options	Sep. 30, 2010 Non-Qualified Options
Share-based compensation arrangement by share-based payment award
Granted (in shares)			1,420,000
Target options outstanding
Opening balance - outstanding Options (in shares)				750,130	879,783	1,015,610	768,652	1,015,610
Forfeitures and cancellations (in shares)				(9,260)			(27,782)	(37,044)
Vested and expired (in shares)					(43,218)			(142,001)
Ending balance - outstanding Options (in shares)				740,870	836,565	879,783	740,870	836,565
Expense Amortized
Accrued expense at the beginning of period	$ 169.9	$ 157.4		$ 4.8	$ 4.1	$ 3.6	$ 4.5	$ 3.6
Expense recognized				0.1	0.2		0.4	0.7				10.1	0.1	11.5
Accrued expense at the end of the period	$ 169.9	$ 157.4		$ 4.9	$ 4.3	$ 4.1	$ 4.9	$ 4.3
Options vesting period (in years)							Third, fourth and fifth anniversaries
Expiration period of award (in years)						5.5
Assumptions of stock-based compensation awards
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)							5.5
Initial exercise price (in dollars per share)			$ 30.00
Amended exercise price of option (in dollars per share)									$ 25.47	$ 27.97
OneBeacon special dividend (in dollars per share)					$ 2.50			$ 2.50			$ 2.03