Loss Reserves - Additional Information (Detail) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2019	Mar. 31, 2018
Causes Of Increase Decrease In Liability For Unpaid Claims And Claims Adjustment Expense [Line Items]
Increase in incurred losses and LAE	$ 84,200
Increase in net prior accident year reserve strengthening	50,400
Provision for Losses and LAE for Claims Occurring in the Current Year	218,029	$ 184,179
Net prior accident year reserve strengthening	52,323	1,966
Increase in losses and loss adjustment expense payments	57,500
US Insurance Reporting Segment
Causes Of Increase Decrease In Liability For Unpaid Claims And Claims Adjustment Expense [Line Items]
Net prior accident year reserve strengthening	26,600
US Insurance Reporting Segment | US Marine Operating Segment
Causes Of Increase Decrease In Liability For Unpaid Claims And Claims Adjustment Expense [Line Items]
Net prior accident year reserve strengthening	23,400
US Insurance Reporting Segment | US Professional Liability Operating Segment
Causes Of Increase Decrease In Liability For Unpaid Claims And Claims Adjustment Expense [Line Items]
Net prior accident year reserve strengthening	2,200
US Insurance Reporting Segment | P&C Operating Segment
Causes Of Increase Decrease In Liability For Unpaid Claims And Claims Adjustment Expense [Line Items]
Net prior accident year reserve strengthening	1,000
Int'l Insurance Reporting Segment
Causes Of Increase Decrease In Liability For Unpaid Claims And Claims Adjustment Expense [Line Items]
Net prior accident year reserve strengthening	16,800
Int'l Insurance Reporting Segment | P&C Operating Segment
Causes Of Increase Decrease In Liability For Unpaid Claims And Claims Adjustment Expense [Line Items]
Net prior accident year reserve strengthening	(800)
Int'l Insurance Reporting Segment | Int'l Professional Liability Operating Segment
Causes Of Increase Decrease In Liability For Unpaid Claims And Claims Adjustment Expense [Line Items]
Net prior accident year reserve strengthening	15,600
Int'l Insurance Reporting Segment | Int'l Marine Operating Segment
Causes Of Increase Decrease In Liability For Unpaid Claims And Claims Adjustment Expense [Line Items]
Net prior accident year reserve strengthening	2,000
Global Reinsurance
Causes Of Increase Decrease In Liability For Unpaid Claims And Claims Adjustment Expense [Line Items]
Net prior accident year reserve strengthening	8,900
Specialty Casualty
Causes Of Increase Decrease In Liability For Unpaid Claims And Claims Adjustment Expense [Line Items]
Net prior accident year reserve strengthening	5,600
P&C Products
Causes Of Increase Decrease In Liability For Unpaid Claims And Claims Adjustment Expense [Line Items]
Net prior accident year reserve strengthening	5,300
Marine Products [Member]
Causes Of Increase Decrease In Liability For Unpaid Claims And Claims Adjustment Expense [Line Items]
Net prior accident year reserve strengthening	1,000
Surety Product
Causes Of Increase Decrease In Liability For Unpaid Claims And Claims Adjustment Expense [Line Items]
Net prior accident year reserve strengthening	(3,000)
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	$ 2,000	$ 0