Employee Share-Based Incentive Compensation Plans (OneBeason Peformance Shares) (Details) (USD $) In Millions, except Share data, unless otherwise specified	12 Months Ended
	Dec. 31, 2012		Dec. 31, 2011		Dec. 31, 2010
Expense Amortized
Accrued expense at the beginning of period	$ 187.9
Accrued expense at the end of the period	159.0
Vesting rate for third fourth and fifth anniversary	33.00%
One Beacon Long Term Incentive Plan
Target options outstanding
Beginning of period (in shares)	642,667		1,464,295		2,224,215
Shares paid or expired (in shares)	(258,901)	[1],[2]	(936,150)	[1],[2]	(889,594)	[1],[2]
New grants (in shares)	181,290		194,900		270,691
Assumed forfeitures and cancellations (in shares)	(1,866)		(80,378)		(141,017)
Ending of period (in shares)	563,190		642,667		1,464,295
Expense Amortized
Accrued expense at the beginning of period	9.7		18.5		15.1
Payments and deferrals	(7.7)	[1],[2]	(10.5)	[1],[2]	(4.6)	[1],[2]
Assumed forfeitures and cancelations	0		(0.5)		(2.2)
Expense recognized	(0.8)		2.2		10.2
Accrued expense at the end of the period	$ 1.2		$ 9.7		$ 18.5
Performance Cycle From 2009 to 2011 | One Beacon Long Term Incentive Plan
Expense Amortized
Percentage of payout for minimum growth target percentage	138.60%
One Beacon Long Term Incentive Plan | Performance Share
Expense Amortized
Target vesting percentage	100.00%
Range of Performance Cycle from 2008 to 2010 | One Beacon Long Term Incentive Plan
Expense Amortized
Percentage of payout for minimum growth target percentage	68.50%
Range of Performance Cycle from 2007 to 2009 | One Beacon Long Term Incentive Plan
Expense Amortized
Percentage of payout for minimum growth target percentage	14.20%

[1] OneBeacon performance share payments in 2010 also include accelerated payments resulting from the OneBeacon Personal Lines and Commercial Lines Transactions to employees of those businesses. 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] 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. OneBeacon performance share payments in 2010 for the 2007-2009 performance cycle were at 14.2% of target. Amounts include deposits into OneBeacon’s deferred compensation plan.