Amortized Cost and Estimated Fair Value of Fixed Maturity Available-for-Sale Securities by Investment Grade (Detail) - USD ($) $ in Thousands	Mar. 31, 2017	Dec. 31, 2016
Schedule of Available-for-sale Securities [Line Items]
Fixed maturity available-for-sale securities, amortized cost	$ 1,354,397	$ 1,383,787
Fixed maturity available-for-sale securities, at estimated fair value	1,463,444	1,487,037
Investment grade
Schedule of Available-for-sale Securities [Line Items]
Fixed maturity available-for-sale securities, amortized cost	1,272,922	1,294,978
Fixed maturity available-for-sale securities, at estimated fair value	1,375,025	1,393,503
Below investment grade
Schedule of Available-for-sale Securities [Line Items]
Fixed maturity available-for-sale securities, amortized cost	81,475	88,809
Fixed maturity available-for-sale securities, at estimated fair value	$ 88,419	$ 93,534