Employee Share-Based Incentive Compensation Plans (Details 2) (OneBeacon's share-based compensation plans, USD $) In Millions, except Share data, unless otherwise specified	1 Months Ended	3 Months Ended	9 Months Ended		1 Months Ended
	Nov. 30, 2006	Sep. 30, 2011	Sep. 30, 2012	Sep. 30, 2011	Nov. 30, 2006 Nonqualified stock options Y	Nov. 16, 2010 Nonqualified stock options	May 27, 2008 Nonqualified stock options
Employee Share-Based Incentive Compensation Plans
Numbers of shares issued to employees	1,420,000
Options vesting period (in years)	third, fourth and fifth anniversaries
Expiration period of award (in years)					five and a half years
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
Additional disclosures
Amended exercise price of option (in dollars per share)						$ 25.47	$ 27.97
OneBeacon special dividend (in dollars per share)						$ 2.50	$ 2.03
Stock Options
Beginning of period (options)		750,130	740,870	768,652
Forfeitures (options)		(9,260)		(27,782)
Vested and expired (options)			(740,870)
End of period (options)		740,870		740,870
Expense amortized
Expense recognized		$ 0.1	$ 5.0	$ 0.4