Summary of Estimated Fair Values of Bonds Held on Deposit (Detail) - USD ($) $ in Thousands		Jun. 30, 2018	Dec. 31, 2017
Debt Securities, Available-for-sale [Line Items]
Estimated Fair Value		$ 483,106	$ 541,428
On Deposit With Governmental Authorities
Debt Securities, Available-for-sale [Line Items]
Estimated Fair Value		25,969	26,852
Intercompany Trusts Held For Benefit Of U.S. Policyholders
Debt Securities, Available-for-sale [Line Items]
Estimated Fair Value		276,687	328,494
Held In Trust Pursuant To Third Party Requirements
Debt Securities, Available-for-sale [Line Items]
Estimated Fair Value		97,650	94,098
Letter Of Credit Held For Third Party Requirements
Debt Securities, Available-for-sale [Line Items]
Estimated Fair Value		2,317	3,944
Securities held as collateral for borrowing arrangements
Debt Securities, Available-for-sale [Line Items]
Estimated Fair Value	[1]	$ 80,483	$ 88,040

[1] Amount required to collateralize margin borrowing facility.