Summary of Reconciliation of Net Incurred and Paid Claims Development to the Reserve for Losses and LAE (Detail) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Shortduration Insurance Contracts Reconciliation Of Claims Development To Liability [Line Items]
Total Net reserves	$ 1,640,874
Reinsurance Recoverables on Unpaid Losses and LAE	809,765	$ 779,276	$ 809,518
Total Gross Reserves	2,450,639
Unallocated Claims Adjustment Expenses	64,269
Other	237
Total Gross Liability for Unpaid Claims and Claim Adjustment Expense	2,515,145	$ 2,289,727	$ 2,202,644
U.S.Insurance | Marine
Shortduration Insurance Contracts Reconciliation Of Claims Development To Liability [Line Items]
Total Net reserves	101,091
Reinsurance Recoverables on Unpaid Losses and LAE	98,373
Total Gross Reserves	199,464
U.S.Insurance | Property Casualty
Shortduration Insurance Contracts Reconciliation Of Claims Development To Liability [Line Items]
Total Net reserves	849,009
Reinsurance Recoverables on Unpaid Losses and LAE	371,651
Total Gross Reserves	1,220,660
U.S.Insurance | Professional Liability
Shortduration Insurance Contracts Reconciliation Of Claims Development To Liability [Line Items]
Total Net reserves	108,302
Reinsurance Recoverables on Unpaid Losses and LAE	49,650
Total Gross Reserves	157,952
International Insurance | Marine
Shortduration Insurance Contracts Reconciliation Of Claims Development To Liability [Line Items]
Total Net reserves	221,659
Reinsurance Recoverables on Unpaid Losses and LAE	90,792
Total Gross Reserves	312,451
International Insurance | Property Casualty
Shortduration Insurance Contracts Reconciliation Of Claims Development To Liability [Line Items]
Total Net reserves	95,680
Reinsurance Recoverables on Unpaid Losses and LAE	126,365
Total Gross Reserves	222,045
International Insurance | Professional Liability
Shortduration Insurance Contracts Reconciliation Of Claims Development To Liability [Line Items]
Total Net reserves	116,783
Reinsurance Recoverables on Unpaid Losses and LAE	55,393
Total Gross Reserves	172,176
Global Reinsurance
Shortduration Insurance Contracts Reconciliation Of Claims Development To Liability [Line Items]
Total Net reserves	148,350
Reinsurance Recoverables on Unpaid Losses and LAE	17,541
Total Gross Reserves	$ 165,891