Amortized Cost and Estimated Fair Value of Fixed Maturity Available-for-Sale Securities by Investment Grade (Detail) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Schedule Of Available For Sale Securities [Line Items]
Fixed maturity available-for-sale securities, amortized cost	$ 1,418,128	$ 1,383,787
Fixed maturity AFS securities, at estimated fair value	1,548,638	1,487,037
Investment grade
Schedule Of Available For Sale Securities [Line Items]
Fixed maturity available-for-sale securities, amortized cost	1,337,738	1,294,978
Fixed maturity AFS securities, at estimated fair value	1,460,708	1,393,503
Below investment grade
Schedule Of Available For Sale Securities [Line Items]
Fixed maturity available-for-sale securities, amortized cost	80,390	88,809
Fixed maturity AFS securities, at estimated fair value	$ 87,930	$ 93,534