Loss and Loss Adjustment Expense Reserves Details (Details) (USD $) In Millions, unless otherwise specified	3 Months Ended		9 Months Ended
	Sep. 30, 2012	Sep. 30, 2011	Sep. 30, 2012	Sep. 30, 2011
Loss and Loss Adjustment, Expense Reserves [Line Items]
Liability for Unpaid Claims and Claims Adjustment Expense, Incurred Claims, Prior Years	$ 10.1	$ 26.9	$ 21.2	$ 47.6
Accretion of fair value adjustment to net loss and LAE reserves	1.1	2.1	9.4	6.2
Accretion of fair value adjustment to net loss and LAE reserves increase			5.0
Remaining unamortized fair value adjustment			2.0
One Beacon Segment [Member]
Loss and Loss Adjustment, Expense Reserves [Line Items]
Liability for Unpaid Claims and Claims Adjustment Expense, Incurred Claims, Prior Years	(2.3)	(6.0)	(7.6)	(14.4)
Sirius
Loss and Loss Adjustment, Expense Reserves [Line Items]
Liability for Unpaid Claims and Claims Adjustment Expense, Incurred Claims, Prior Years	(7.8)	(20.9)	(13.6)	(33.2)
Net Unfavorable Loss Reserve Development Due To Asbestos And Environmental Reserves Additions	$ 33.0		$ 45.0