Summary of Reconciliation of Net Incurred and Paid Claims Development to the Reserve for Losses and LAE (Detail) - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Shortduration Insurance Contracts Reconciliation Of Claims Development To Liability [Line Items]
Total Net Reserves	$ 1,835,135
Reinsurance Recoverables on Unpaid Claims	804,633
Total Gross Reserves	2,639,768
Unallocated Claims Adjustment Expenses	72,099
ASCO Reserves	31,834
Other	272
Total Gross Liability for Unpaid Claims and Claim Adjustment Expense	2,743,973	$ 2,515,145	$ 2,289,727
U.S.Insurance | Professional Liability
Shortduration Insurance Contracts Reconciliation Of Claims Development To Liability [Line Items]
Total Net Reserves	128,321
Reinsurance Recoverables on Unpaid Claims	54,052
Total Gross Reserves	182,373
U.S.Insurance | Marine
Shortduration Insurance Contracts Reconciliation Of Claims Development To Liability [Line Items]
Total Net Reserves	90,028
Reinsurance Recoverables on Unpaid Claims	88,783
Total Gross Reserves	178,811
U.S.Insurance | Property Casualty
Shortduration Insurance Contracts Reconciliation Of Claims Development To Liability [Line Items]
Total Net Reserves	977,968
Reinsurance Recoverables on Unpaid Claims	387,081
Total Gross Reserves	1,365,049
International Insurance | Professional Liability
Shortduration Insurance Contracts Reconciliation Of Claims Development To Liability [Line Items]
Total Net Reserves	162,600
Reinsurance Recoverables on Unpaid Claims	86,970
Total Gross Reserves	249,570
International Insurance | Marine
Shortduration Insurance Contracts Reconciliation Of Claims Development To Liability [Line Items]
Total Net Reserves	214,455
Reinsurance Recoverables on Unpaid Claims	68,117
Total Gross Reserves	282,572
International Insurance | Property Casualty
Shortduration Insurance Contracts Reconciliation Of Claims Development To Liability [Line Items]
Total Net Reserves	87,368
Reinsurance Recoverables on Unpaid Claims	106,879
Total Gross Reserves	194,247
Global Reinsurance
Shortduration Insurance Contracts Reconciliation Of Claims Development To Liability [Line Items]
Total Net Reserves	174,395
Reinsurance Recoverables on Unpaid Claims	12,751
Total Gross Reserves	$ 187,146