Loss and Loss Adjustment Expense Reserves (Loss and Loss Adjustment Expense) (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	Mar. 31, 2012	Mar. 31, 2011
Loss and Loss Adjustment Expense Reserve
Gross beginning balance	$ 5,329.8	$ 5,642.4	$ 5,702.3	$ 5,736.8
Less beginning reinsurance recoverable on unpaid losses			(2,507.3)	(2,344.0)	(2,369.4)	(2,193.0)
Net loss and LAE reserves			3,195.0	3,392.8	2,960.4	3,449.4
Loss and LAE incurred relating to current year losses	318.2	307.1	842.9	968.0
Net favorable and unfavorable loss reserve development	(10.1)	(26.9)	(21.2)	(47.6)
Total incurred losses and LAE	308.1	280.2	821.7	920.4
Accretion of fair value adjustment to net loss and LAE reserves	1.1	2.1	9.4	6.2
Foreign currency translation adjustment to net loss and LAE reserves	12.0	(25.5)	11.7	5.7
Current year losses	(101.1)	(109.7)	(223.9)	(226.3)
Prior year losses	(181.5)	(186.9)	(726.8)	(571.8)
Total loss and LAE payments	(282.6)	(296.6)	(950.7)	(798.1)
Net loss and LAE reserves	2,703.0	3,300.8	2,703.0	3,300.8	2,960.4	3,449.4
Plus ending reinsurance recoverable on unpaid losses	356.8	2,581.2	356.8	2,581.2	2,369.4	2,193.0
Gross ending balance	3,059.8	5,882.0	3,059.8	5,882.0
One Beacon
Loss and Loss Adjustment Expense Reserve
Net favorable and unfavorable loss reserve development	2.3	6.0	7.6	14.4
Sirius
Loss and Loss Adjustment Expense Reserve
Net favorable and unfavorable loss reserve development	7.8	20.9	13.6	33.2
Net Unfavorable Loss Reserve Development Due To Asbestos And Environmental Reserves Additions	33.0		45.0
Total incurred losses and LAE	143.4	130.5	369.2	499.1
Plus ending reinsurance recoverable on unpaid losses	315.5		315.5
Segment, Discontinued Operations [Member]
Loss and Loss Adjustment Expense Reserve
Gross beginning balance			64.7
Less beginning reinsurance recoverable on unpaid losses			0
Net loss and LAE reserves	(296.0)	(503.2)	(384.1)	(620.6)
Total incurred losses and LAE	27.7	177.4	48.4	512.3
Net loss and LAE reserves	0	394.4	0	394.4
Plus ending reinsurance recoverable on unpaid losses	1,956.6		1,956.6
Gross ending balance	$ 2,212.9		$ 2,212.9