Liability for Unpaid Claims and Claim Adjustment Expenses - Summary of Incurred and Paid Claims Development as Supplementary Information - Short Tail Property (Detail) $ in Thousands	Dec. 31, 2019 USD ($) claim	Dec. 31, 2018 USD ($)
Claims Development [Line Items]
Liabilities for claims and claim adjustment expenses, net of reinsurance	$ 995,986
Short Tail Property
Claims Development [Line Items]
Incurred claims and allocated claim adjustment expenses, net of reinsurance	491,042
Cumulative paid claims and allocated claim adjustment expenses, net of reinsurance	460,051
All outstanding liabilities before period one, net of reinsurance	2,106
Liabilities for claims and claim adjustment expenses, net of reinsurance	33,097
Short Tail Property | Accident Year 2018
Claims Development [Line Items]
Incurred claims and allocated claim adjustment expenses, net of reinsurance	243,813	$ 248,182
IBNR plus expected development	$ 208
Cumulative number of reported claims | claim	66,743
Cumulative paid claims and allocated claim adjustment expenses, net of reinsurance	$ 241,718	$ 218,095
Short Tail Property | Accident Year 2019
Claims Development [Line Items]
Incurred claims and allocated claim adjustment expenses, net of reinsurance	247,229
IBNR plus expected development	$ (2,442)
Cumulative number of reported claims | claim	62,401
Cumulative paid claims and allocated claim adjustment expenses, net of reinsurance	$ 218,333