Reserves for Losses and LAE - Additional Information (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Causes Of Increase Decrease In Liability For Unpaid Claims And Claims Adjustment Expense [Line Items]
Increase in incurred losses and LAE	$ 87,500	$ 140,800
Provision for Losses and LAE for Claims Occurring in the Current Year	817,537	771,955	$ 693,976
Net reserve deficiencies (redundancies) in prior years	76,242	34,310	(28,528)
Increase in losses and LAE payments	56,400	91,000
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	18,908	16,714	(1,348)
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	9,387	13,405	5,984
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	10,310	1,613	2,790
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	(789)	1,696	(10,122)
International Insurance
Causes Of Increase Decrease In Liability For Unpaid Claims And Claims Adjustment Expense [Line Items]
Net reserve deficiencies (redundancies) in prior years	31,353	14,834	(25,019)
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	14,478	5,198	4,100
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	(4,913)	3,118	(7,051)
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	21,788	6,518	(22,068)
Global Reinsurance
Causes Of Increase Decrease In Liability For Unpaid Claims And Claims Adjustment Expense [Line Items]
Net reserve deficiencies (redundancies) in prior years	25,981	2,762	(2,161)
Typhoon Jebi
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	13,900
Hurricane Michael
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	4,800
Hurricane Florence
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
California Camp Fire
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	3,200
Typhoon Trami
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	3,000
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	300	9,600
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	28,700	84,800
Short-duration Insurance Contracts Incurred But Not Reported IBNR Claims And Recoveries In Excess Of Loss	14,200
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	1,800	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	900	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	900	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	3,500	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	1,900	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	1,600	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	$ 23,400	43,700
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
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