Employee Share-Based Incentive Compensation Plans (Details 5) (USD $) In Millions, except Share data, unless otherwise specified			3 Months Ended				6 Months Ended								1 Months Ended	3 Months Ended		6 Months Ended
	Jun. 30, 2012	Dec. 31, 2011	Jun. 30, 2012 OneBeacon Long-Term Incentive Plan Performance Share [Member]		Jun. 30, 2011 OneBeacon Long-Term Incentive Plan Performance Share [Member]		Jun. 30, 2012 OneBeacon Long-Term Incentive Plan Performance Share [Member]		Jun. 30, 2011 OneBeacon Long-Term Incentive Plan Performance Share [Member]		Mar. 31, 2012 OneBeacon Long-Term Incentive Plan Performance Share [Member]	Dec. 31, 2011 OneBeacon Long-Term Incentive Plan Performance Share [Member]	Mar. 31, 2011 OneBeacon Long-Term Incentive Plan Performance Share [Member]	Dec. 31, 2010 OneBeacon Long-Term Incentive Plan Performance Share [Member]	Nov. 30, 2006 OneBeacon Long-Term Incentive Plan Non-Qualified Options	Jun. 30, 2012 OneBeacon Long-Term Incentive Plan Non-Qualified Options	Jun. 30, 2011 OneBeacon Long-Term Incentive Plan Non-Qualified Options	Jun. 30, 2012 OneBeacon Long-Term Incentive Plan Non-Qualified Options Y	Jun. 30, 2011 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
Share-based compensation arrangement by share-based payment award
Share-based Compensation Arrangement by Share-based Payment Award, Options, Grants in Period, Gross																0	0	0	0
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Target Shares Paid in Period			(9,168)	[1],[2]	0	[1],[2]	(258,901)	[1],[2]	(936,150)	[1],[2]
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Nonvested, Number			560,577		671,727		560,577		671,727		569,745	642,667	718,172	1,464,295
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)																		5.5
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		$ 2.03
Target options outstanding
Ending balance - outstanding Options (in shares)																0	750,130	0	750,130
Opening balance - outstanding Options (in shares)																740,870	768,652	740,870	768,652
Expense Amortized
Accrued expense at the beginning of period	$ 113.6	$ 187.9	$ 3.2		$ 8.4		$ 9.7		$ 18.5							$ 5.0	$ 4.6	$ 5.0	$ 4.5
Expense recognized			0.2		1.9		1.2		2.3							0	0.2	0	0.3
Accrued expense at the end of the period	113.6	187.9	3.1		10.0		3.1		10.0							5.0	4.8	5.0	4.8
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Amounts Paid in Period			(0.3)	[1],[2]	0	[1],[2]	(7.8)	[1],[2]	(10.5)	[1],[2]
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Grants in Period			0		0		181,290		194,900
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Forfeited in Period			0	[3]	(46,445)	[3]	(4,479)	[3]	(51,318)	[3]
Employee Service Share-based Compensation Nonvested Awards Forfeited in Period Total Compensation Cost Not yet Recognized			$ 0	[3]	$ (0.3)	[3]	$ 0	[3]	$ (0.3)	[3]
Share-based Compensation Arrangement by Share-based Payment Award, Options, Forfeitures in Period																0	(18,522)	0	(18,522)
Share-based Compensation Arrangement by Share-based Payment Award, Options Vested and Expirations in Period																(740,870)	0	(740,870)	0
Share-based Compensation Arrangement by Share-based Payment Award, Options, Exercises in Period																0	0	0	0

[1] (2) OneBeacon performance share payments also include accelerated payments resulting from the OneBeacon Personal Lines and Commercial Lines Transactions. The accelerated OneBeacon performance shares payments for the 2009-2011 and 2010-2012 performance cycles were on a pro rata basis and at a performance factor of 100%.

[2] (1) OneBeacon performance share payments in 2012 for the 2009-2011 performance cycle were at 138.6% of target. OneBeacon performance shares payments in 2011 for the 2008-2010 performance cycle were at 68.5% of target. Amounts include deposits into OneBeacon’s deferred compensation plan.

[3] (3) Amounts include changes in assumed forfeitures, as required under GAAP.