Summary of Estimated Fair Values of Bonds Held on Deposit (Detail) - USD ($) $ in Thousands	Sep. 30, 2021	Dec. 31, 2020
Schedule Of Available For Sale Securities [Line Items]
Estimated Fair Value	$ 108,927	$ 131,664
On Deposit With Governmental Authorities
Schedule Of Available For Sale Securities [Line Items]
Estimated Fair Value	26,468	26,966
Held In Trust Pursuant To Third Party Requirements
Schedule Of Available For Sale Securities [Line Items]
Estimated Fair Value	79,947	100,234
Letter Of Credit Held For Third Party Requirements
Schedule Of Available For Sale Securities [Line Items]
Estimated Fair Value	2,512	3,970
Securities Held As Collateral
Schedule Of Available For Sale Securities [Line Items]
Estimated Fair Value	$ 0	$ 494