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Losses and Loss Expense Reserve - Summary of Balance of Subrogation Recoveries and Related Claim Liabilities, by Estimation Approach (Detail) (USD $) In Thousands, unless otherwise specified	Sep. 30, 2012 Trial	Dec. 31, 2011 Trial
Schedule Of Balance Of Subrogation Recoveries And Related Claim Liabilities By Estimation Approach [Line Items]
Count	44	46
Gross claim liability before subrogation recoveries	$ 3,891,776	$ 3,777,581
Subrogation recoveries(1)	(2,688,867)	(2,720,266)
Gross claim liability after subrogation recoveries	1,202,909	1,057,315
Adverse Samples [Member]
Schedule Of Balance Of Subrogation Recoveries And Related Claim Liabilities By Estimation Approach [Line Items]
Count	28	30
Gross claim liability before subrogation recoveries	2,734,471	2,637,479
Subrogation recoveries(1)	(1,508,150)	(1,457,472)
Gross claim liability after subrogation recoveries	1,226,321	1,180,007
Random Samples [Member]
Schedule Of Balance Of Subrogation Recoveries And Related Claim Liabilities By Estimation Approach [Line Items]
Count	16	16
Gross claim liability before subrogation recoveries	1,157,305	1,140,102
Subrogation recoveries(1)	(1,180,717)	(1,262,794)
Gross claim liability after subrogation recoveries	$ (23,412)	$ (122,692)