Reserves for Losses and LAE - Additional Information (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Net reserve deficiencies (redundancies) in prior years	$ (64,669)	$ (55,812)	$ (1,266)
U.S.Insurance
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Net reserve deficiencies (redundancies) in prior years	(29,319)	(15,420)	11,036
U.S.Insurance | Marine
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Net reserve deficiencies (redundancies) in prior years	(24,825)	(29,047)	(13,679)
U.S.Insurance | Marine | Marine Liability Business
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Net reserve deficiencies (redundancies) in prior years			(13,700)
U.S.Insurance | Property Casualty
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Net reserve deficiencies (redundancies) in prior years	(706)	17,081	12,938
U.S.Insurance | Professional Liability
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Net reserve deficiencies (redundancies) in prior years	(3,788)	(3,454)	11,777
U.S.Insurance | Professional Liability | Errors and Omissions
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Net reserve deficiencies (redundancies) in prior years	(3,788)
U.S.Insurance | Professional Liability | Directors and Officers
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Net reserve deficiencies (redundancies) in prior years		(3,500)	11,800
International Insurance
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Net reserve deficiencies (redundancies) in prior years	(26,216)	(38,252)	(27,335)
International Insurance | Marine
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Net reserve deficiencies (redundancies) in prior years	(21,914)	(31,677)	(6,361)
International Insurance | Marine | Marine Liability Business
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Net reserve deficiencies (redundancies) in prior years			(6,400)
International Insurance | Property Casualty
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Net reserve deficiencies (redundancies) in prior years	(8,458)	(3,829)	(22,263)
International Insurance | Property Casualty | Marine Liability Business
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Net reserve deficiencies (redundancies) in prior years			(22,300)
International Insurance | Professional Liability
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Net reserve deficiencies (redundancies) in prior years	4,156	(2,746)	1,289
International Insurance | Professional Liability | Errors and Omissions
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Net reserve deficiencies (redundancies) in prior years			1,300
International Insurance | Professional Liability | Directors and Officers
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Net reserve deficiencies (redundancies) in prior years	4,200
International Insurance | Professional Liability | Errors And Omissions And Management Liability
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Net reserve deficiencies (redundancies) in prior years		(2,700)
Global Reinsurance
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Net reserve deficiencies (redundancies) in prior years	(9,134)	(2,140)	15,033
Global Reinsurance | Marine
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Net reserve deficiencies (redundancies) in prior years		$ (2,140)	$ 15,000
Global Reinsurance | Marine Property And Casualty And Accident And Health
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Net reserve deficiencies (redundancies) in prior years	$ (9,134)