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Summary of Estimated Fair Values of Bonds Held on Deposit (Detail) - USD ($) $ in Thousands		Mar. 31, 2018	Dec. 31, 2017
Schedule of Available-for-sale Securities [Line Items]
Estimated Fair Value		$ 495,949	$ 541,428
On Deposit With Governmental Authorities
Schedule of Available-for-sale Securities [Line Items]
Estimated Fair Value		26,412	26,852
Intercompany Trusts Held For Benefit Of U.S. Policyholders
Schedule of Available-for-sale Securities [Line Items]
Estimated Fair Value		291,974	328,494
Held In Trust Pursuant To Third Party Requirements
Schedule of Available-for-sale Securities [Line Items]
Estimated Fair Value		96,803	94,098
Letter Of Credit Held For Third Party Requirements
Schedule of Available-for-sale Securities [Line Items]
Estimated Fair Value		2,707	3,944
Securities held as collateral for borrowing arrangements
Schedule of Available-for-sale Securities [Line Items]
Estimated Fair Value	[1]	$ 78,053	$ 88,040
[1] Amount required to collateralize margin borrowing facility.

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Fair Value Of Securities On Deposit With Various Regulatory Authorities

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