Employee Share-Based Incentive Compensation Plans (OneBeason - Peformance Shares) (Details) (USD $) In Millions, except Share data, unless otherwise specified			3 Months Ended
	Mar. 31, 2013	Dec. 31, 2012	Mar. 31, 2013 Performance Share One Beacon Long Term Incentive Plan		Mar. 31, 2012 Performance Share One Beacon Long Term Incentive Plan		Mar. 31, 2012 Performance cycle 2009-2011 Performance Share One Beacon Long Term Incentive Plan
Target Performance Shares Outstanding
Beginning of period (in shares)			563,190		642,667
Shares paid or expired (in shares)			(238,658)	[1]	(249,733)	[1]
New grants (in shares)			179,000		181,290
Assumed forfeitures and cancellations			(13,665)	[2]	(4,479)	[2]
Ending of period (in shares)			489,867		569,745
Expense Amortized
Accrued expense at the beginning of period	$ 100.8	$ 159.0	$ 1.2		$ 9.7
Payments and deferrals			0	[1]	(7.5)	[1]
Assumed forfeitures and cancellations			0	[2]	0	[2]
Expense recognized			0.4		1.0
Accrued expense at the end of the period	100.8	159.0	1.6		3.2
Percentage of payout for minimum growth target percentage							138.60%
Additional compensation cost that would be recognized if all outstanding performance shares vested			$ 1.4

[1] There were no payments made in 2013 for the 2010-2012 performance cycle; those performance shares did not meet the threshold performance goals and expired. OneBeacon performance share payments in 2012 for the 2009-2011 performance cycle were at 138.6% of target. Amounts include deposits into OneBeacon’s deferred compensation plan.

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