Amortized Cost and Estimated Fair Value of Fixed Maturity Available-for-Sale Securities by Investment Grade (Detail) - USD ($) $ in Thousands	Mar. 31, 2018	Dec. 31, 2017
Schedule of Available-for-sale Securities [Line Items]
Fixed maturity available-for-sale securities, amortized cost	$ 1,219,443	$ 1,418,128
Fixed maturity AFS securities, at estimated fair value	1,294,321	1,548,638
Fixed maturity available-for-sale securities, at estimated fair value	1,294,321	1,548,638
Investment grade
Schedule of Available-for-sale Securities [Line Items]
Fixed maturity available-for-sale securities, amortized cost	1,156,451	1,337,738
Fixed maturity AFS securities, at estimated fair value	1,225,374	1,460,708
Fixed maturity available-for-sale securities, at estimated fair value	1,225,374	1,460,708
Below investment grade
Schedule of Available-for-sale Securities [Line Items]
Fixed maturity available-for-sale securities, amortized cost	62,992	80,390
Fixed maturity AFS securities, at estimated fair value	68,947	87,930
Fixed maturity available-for-sale securities, at estimated fair value	$ 68,947	$ 87,930