Employee Share-Based Incentive Compensation Plans (OneBeason Peformance Shares) (Details) (USD $) In Millions, except Share data, unless otherwise specified			3 Months Ended				9 Months Ended						9 Months Ended
	Sep. 30, 2012	Dec. 31, 2011	Sep. 30, 2012 One Beacon Long Term Incentive Plan [Member] Performance Share [Member]		Sep. 30, 2011 One Beacon Long Term Incentive Plan [Member] Performance Share [Member]		Sep. 30, 2012 One Beacon Long Term Incentive Plan [Member] Performance Share [Member]		Sep. 30, 2011 One Beacon Long Term Incentive Plan [Member] Performance Share [Member]		Sep. 30, 2012 Performance cycle 2009-2011 One Beacon Long Term Incentive Plan [Member] Performance Share [Member]	Sep. 30, 2012 One Beacon Long Term Incentive Plan [Member] Performance Share [Member]	Sep. 30, 2012 Range of Performance Cycle from 2008 to 2010 [Member] One Beacon Long Term Incentive Plan [Member] Performance Share [Member]
Target options outstanding
Beginning of period (in shares)			560,577		671,727		642,667		1,464,295
Shares paid or expired (in shares)			0	[1],[2]	0	[1],[2]	(258,901)	[1],[2]	(936,150)	[1],[2]
New grants (in shares)			0		0		181,290		194,900
Assumed forfeitures and cancellations			3,354	[3]	(19,583)	[3]	(7,833)	[3]	(70,901)	[3]
Ending of period (in shares)			557,223		652,144		557,223		652,144
Expense Amortized
Accrued expense at the beginning of period	$ 144.7	$ 187.9	$ 3.1		$ 10.0		$ 9.7		$ 18.5
Payments and deferrals			0	[1],[2]	0	[1],[2]	(7.8)	[1],[2]	(10.5)	[1],[2]
Assumed forfeitures and cancelations			0	[3]	0.1	[3]	0	[3]	(0.2)	[3]
Expense recognized			(2.2)		(0.5)		(1.0)		1.8
Accrued expense at the end of the period	144.7	187.9	0.9		9.6		0.9		9.6
Percentage of payout for minimum growth target percentage											138.60%		68.50%
Performance Share Target Vesting Percentage												100.00%
Additional compensation cost that would be recognized if all outstanding performance shares vested			$ 1.7				$ 1.7

[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.