Loss and Loss Adjustment Expense Reserves Details (Details) (USD $) In Millions, unless otherwise specified	3 Months Ended
	Mar. 31, 2013	Mar. 31, 2012
Loss and Loss Adjustment, Expense Reserves [Line Items]
Net favorable and unfavorable loss reserve development	$ 1.4	$ (7.6)
Liability For Unpaid Claims And Claims Adjustment Expense Incurred Claims Prior Years Due To 2011 Japan Earthquake		8.1
Accretion of fair value adjustment to net loss and LAE reserves	1.2	7.1
Adjustment to loss and loss adjustment expense reserves, before tax	4.5
One Beacon
Loss and Loss Adjustment, Expense Reserves [Line Items]
Net favorable and unfavorable loss reserve development	(2.9)	(1.9)
Sirius
Loss and Loss Adjustment, Expense Reserves [Line Items]
Net favorable and unfavorable loss reserve development	4.3	(5.7)
Scandinavian Reinsurance Company [Member]
Loss and Loss Adjustment, Expense Reserves [Line Items]
Accretion of fair value adjustment to net loss and LAE reserves increase		$ 5.0