Reporting and Operating Segments Breakdowns of Prior Period Net Reserve Deficiency (Redundancy) (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Liability for Claims and Claims Adjustment Expense [Line Items]
Net reserve deficiencies (redundancies) in prior years	$ 34,310	$ (28,528)	$ (64,669)
U.S.Insurance
Liability for Claims and Claims Adjustment Expense [Line Items]
Net reserve deficiencies (redundancies) in prior years	16,714	(1,348)	(29,319)
U.S.Insurance | Marine
Liability for Claims and Claims Adjustment Expense [Line Items]
Net reserve deficiencies (redundancies) in prior years	1,696	(10,122)	(24,825)
U.S.Insurance | Property Casualty
Liability for Claims and Claims Adjustment Expense [Line Items]
Net reserve deficiencies (redundancies) in prior years	1,613	2,790	(706)
U.S.Insurance | Professional Liability
Liability for Claims and Claims Adjustment Expense [Line Items]
Net reserve deficiencies (redundancies) in prior years	13,405	5,984	(3,788)
International Insurance
Liability for Claims and Claims Adjustment Expense [Line Items]
Net reserve deficiencies (redundancies) in prior years	14,834	(25,019)	(26,216)
International Insurance | Marine
Liability for Claims and Claims Adjustment Expense [Line Items]
Net reserve deficiencies (redundancies) in prior years	6,518	(22,068)	(21,914)
International Insurance | Property Casualty
Liability for Claims and Claims Adjustment Expense [Line Items]
Net reserve deficiencies (redundancies) in prior years	3,118	(7,051)	(8,458)
International Insurance | Professional Liability
Liability for Claims and Claims Adjustment Expense [Line Items]
Net reserve deficiencies (redundancies) in prior years	5,198	4,100	4,156
Global Reinsurance
Liability for Claims and Claims Adjustment Expense [Line Items]
Net reserve deficiencies (redundancies) in prior years	$ 2,762	$ (2,161)	$ (9,134)