Activity in Liability for Loss and Loss Adjustment Expense (Detail) (USD $) In Millions, unless otherwise specified	9 Months Ended
	Sep. 30, 2012		Sep. 30, 2011
Liability for Unpaid Claims and Claims Adjustment Expense, Adjustment of Opening Balance [Line Items]
Reserves, gross of reinsurance recoverables, beginning balance	$ 2,313.0		$ 2,328.7
Less: reinsurance recoverables	831.8	[1]	847.4	[1]
Reserves, net of reinsurance recoverables, beginning balance	1,481.2		1,481.3
Reserves acquired	9,156.1	[2]
Incurred loss, net of reinsurance, related to:
Current year	1,594.6	[3]	344.9	[3]
Prior years	(6.8)		(29.5)
Total incurred loss and LAE, net of reinsurance	1,587.8		315.4
Paid loss, net of reinsurance, related to:
Current year	1,324.4	[3]	52.3	[3]
Prior years	285.1		257.0
Total paid loss and LAE, net of reinsurance	1,609.5		309.3
Foreign exchange effect	(17.1)	[3]
Reserves, net of reinsurance recoverables, ending balance	10,598.5		1,487.4
Reinsurance recoverables, ending balance	1,232.3	[1]	840.8	[1]
Reserves, gross of reinsurance recoverables, ending balance	$ 11,830.8		$ 2,328.2

[1] Reinsurance recoverables in this table include only ceded loss reserves.

[2] Represents the carrying value of Transatlantic's net reserves acquired in the merger. Gross loss reserves and ceded loss reserves as of the Acquisition Date were $9,627.8 million and $471.7 million, respectively.

[3] Includes amounts from Transatlantic from the Acquisition Date through September 30, 2012.