Employee Share-Based Incentive Compensation Plans (OneBeason Peformance Shares) (Details) (USD $) In Millions, except Share data, unless otherwise specified	12 Months Ended
	Dec. 31, 2013		Dec. 31, 2012		Dec. 31, 2011
Target options outstanding
Assumed forfeitures and cancellations (in shares)					0
Expense Amortized
Accrued expense at the end of the period	$ 218.3		$ 159.0
One Beacon Insurance Company | Performance Shares [Member] | Long Term Incentive Plan
Target options outstanding
Beginning of period (in shares)	563,190		642,667		1,464,295
Shares paid or expired (in shares)	(238,658)	[1]	(258,901)	[1]	(936,150)	[1]
New grants (in shares)	179,000		181,290		194,900
Assumed forfeitures and cancellations (in shares)	(10,111)	[2]	(1,866)	[2]	(80,378)	[2]
Ending of period (in shares)	493,421		563,190		642,667
Expense Amortized
Accrued expense at the beginning of period	1.2		9.7		18.5
Payments and deferrals	0	[1]	(7.7)	[1]	(10.5)	[1]
Assumed forfeitures and cancellations	(0.1)	[2]	0	[2]	(0.5)	[2]
Expense recognized	2.9		(0.8)		2.2
Accrued expense at the end of the period	$ 4.0		$ 1.2		$ 9.7
Range of Performance Cycle from 2008 to 2010 | Long Term Incentive Plan
Expense Amortized
Percentage of payout for minimum growth target percentage					68.50%
Performance Cycle From 2009 to 2011 | Long Term Incentive Plan
Expense Amortized
Percentage of payout for minimum growth target percentage			138.60%

[1] No payments were made in 2013 for the 2010-2012 OneBeacon performance cycle as the performance factor was zero. 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.

[2] Amounts include changes in assumed forfeitures, as required under GAAP.