Reserves for Losses and LAE - Additional Information (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Increase in incurred losses and LAE	$ 140,800	$ 92,900
Provision for Losses and LAE for Claims Occurring in the Current Year	771,955	693,976	$ 637,267
Net reserve deficiencies (redundancies) in prior years	34,310	(28,528)	(64,669)
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	16,714	(1,348)	(29,319)
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	1,613	2,790	(706)
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	1,696	(10,122)	(24,825)
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	13,405	5,984	(3,788)
International Insurance
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Net reserve deficiencies (redundancies) in prior years	14,834	(25,019)	(26,216)
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	3,118	(7,051)	(8,458)
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	6,518	(22,068)	(21,914)
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	5,198	4,100	4,156
Global Reinsurance
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Net reserve deficiencies (redundancies) in prior years	2,762	(2,161)	$ (9,134)
Irma
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Provision for Losses and LAE for Claims Occurring in the Current Year	28,300
Maria
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Provision for Losses and LAE for Claims Occurring in the Current Year	23,400
Harvey
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Provision for Losses and LAE for Claims Occurring in the Current Year	15,800
Typhoon Hato
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Provision for Losses and LAE for Claims Occurring in the Current Year	4,200
Puebla, Mexico Earthquake
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Provision for Losses and LAE for Claims Occurring in the Current Year	3,500
Other CAT Events
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Provision for Losses and LAE for Claims Occurring in the Current Year	9,600
Alberta Wildfires
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Provision for Losses and LAE for Claims Occurring in the Current Year		11,700
Hurricane Matthew
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Provision for Losses and LAE for Claims Occurring in the Current Year		7,100
Ecuador Earthquake
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Provision for Losses and LAE for Claims Occurring in the Current Year		3,800
Taiwan Earthquake
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Provision for Losses and LAE for Claims Occurring in the Current Year		3,300
Catastrophe
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Provision for Losses and LAE for Claims Occurring in the Current Year	84,800	26,800
Net incurred but not reported (“IBNR”) claims	18,500
Catastrophe | U.S.Insurance
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Provision for Losses and LAE for Claims Occurring in the Current Year	8,200
Catastrophe | U.S.Insurance | Property Casualty
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Provision for Losses and LAE for Claims Occurring in the Current Year	6,600
Catastrophe | U.S.Insurance | Marine
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Provision for Losses and LAE for Claims Occurring in the Current Year	1,600
Catastrophe | International Insurance
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Provision for Losses and LAE for Claims Occurring in the Current Year	32,900
Catastrophe | International Insurance | Property Casualty
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Provision for Losses and LAE for Claims Occurring in the Current Year	16,900
Catastrophe | International Insurance | Marine
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Provision for Losses and LAE for Claims Occurring in the Current Year	16,000
Catastrophe | Global Reinsurance
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Provision for Losses and LAE for Claims Occurring in the Current Year	$ 43,700
Hurricane Matthew And Alberta Wildfires | International Insurance
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Provision for Losses and LAE for Claims Occurring in the Current Year		17,400
Alberta Wildfires And Ecuador Earthquake | Global Reinsurance
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Provision for Losses and LAE for Claims Occurring in the Current Year		$ 9,400