Activity in Liability for Loss and Loss Adjustment Expense (Parenthetical) (Detail) (USD $) In Millions, unless otherwise specified	Sep. 30, 2012		Dec. 31, 2011		Sep. 30, 2011		Dec. 31, 2010		Mar. 06, 2012 Transatlantic
Liability for Unpaid Claims and Claims Adjustment Expense, Adjustment of Opening Balance [Line Items]
Gross loss reserves as of acquisition date	$ 11,830.8		$ 2,313.0		$ 2,328.2		$ 2,328.7		$ 9,627.8
Ceded loss reserves as of acquisition date	$ 1,232.3	[1]	$ 831.8	[1]	$ 840.8	[1]	$ 847.4	[1]	$ 471.7

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