Summary of Estimated Fair Values of Bonds Held on Deposit (Detail) - USD ($) $ in Thousands		Sep. 30, 2017	Dec. 31, 2016
Schedule of Available-for-sale Securities [Line Items]
Estimated Fair Value		$ 542,517	$ 559,069
On Deposit With Governmental Authorities
Schedule of Available-for-sale Securities [Line Items]
Estimated Fair Value		28,489	29,079
Intercompany Trusts Held For Benefit Of U.S. Policyholders
Schedule of Available-for-sale Securities [Line Items]
Estimated Fair Value		335,444	351,002
Held In Trust Pursuant To Third Party Requirements
Schedule of Available-for-sale Securities [Line Items]
Estimated Fair Value		80,820	88,178
Letter Of Credit Held For Third Party Requirements
Schedule of Available-for-sale Securities [Line Items]
Estimated Fair Value		4,028	4,871
Securities held as collateral for borrowing arrangements
Schedule of Available-for-sale Securities [Line Items]
Estimated Fair Value	[1]	$ 93,736	$ 85,939

[1] Amount required to collateralize margin borrowing facility.