Summary of Reconciliation of Net Incurred and Paid Claims Development to the Reserve for Losses and LAE (Detail) - USD ($) $ in Thousands	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Shortduration Insurance Contracts Reconciliation Of Claims Development To Liability [Line Items]
Total Net reserves	$ 1,454,728
Reinsurance recoverables on unpaid claims	779,276
Total Gross reserves	2,234,004
Unallocated claims adjustment expenses	55,444
Other	279
Total gross liability for unpaid claims and claim adjustment expense	2,289,727	$ 2,202,644	$ 2,159,634
U.S.Insurance | Marine
Shortduration Insurance Contracts Reconciliation Of Claims Development To Liability [Line Items]
Total Net reserves	106,288
Reinsurance recoverables on unpaid claims	83,988
Total Gross reserves	190,276
U.S.Insurance | Property Casualty
Shortduration Insurance Contracts Reconciliation Of Claims Development To Liability [Line Items]
Total Net reserves	769,399
Reinsurance recoverables on unpaid claims	409,648
Total Gross reserves	1,179,047
U.S.Insurance | Professional Liability
Shortduration Insurance Contracts Reconciliation Of Claims Development To Liability [Line Items]
Total Net reserves	91,037
Reinsurance recoverables on unpaid claims	59,938
Total Gross reserves	150,975
International Insurance | Marine
Shortduration Insurance Contracts Reconciliation Of Claims Development To Liability [Line Items]
Total Net reserves	196,532
Reinsurance recoverables on unpaid claims	88,900
Total Gross reserves	285,432
International Insurance | Property Casualty
Shortduration Insurance Contracts Reconciliation Of Claims Development To Liability [Line Items]
Total Net reserves	81,189
Reinsurance recoverables on unpaid claims	79,548
Total Gross reserves	160,737
International Insurance | Professional Liability
Shortduration Insurance Contracts Reconciliation Of Claims Development To Liability [Line Items]
Total Net reserves	98,689
Reinsurance recoverables on unpaid claims	52,518
Total Gross reserves	151,207
Global Reinsurance
Shortduration Insurance Contracts Reconciliation Of Claims Development To Liability [Line Items]
Total Net reserves	111,594
Reinsurance recoverables on unpaid claims	4,736
Total Gross reserves	$ 116,330