Amortized Cost and Estimated Fair Value of Fixed Maturity Available-for-Sale Securities by Investment Grade (Detail) - USD ($) $ in Thousands	Jun. 30, 2017	Dec. 31, 2016
Schedule of Available-for-sale Securities [Line Items]
Fixed maturity available-for-sale securities, amortized cost	$ 1,347,918	$ 1,383,787
Fixed maturity available-for-sale securities, at estimated fair value	1,474,826	1,487,037
Investment grade
Schedule of Available-for-sale Securities [Line Items]
Fixed maturity available-for-sale securities, amortized cost	1,268,702	1,294,978
Fixed maturity available-for-sale securities, at estimated fair value	1,386,926	1,393,503
Below investment grade
Schedule of Available-for-sale Securities [Line Items]
Fixed maturity available-for-sale securities, amortized cost	79,216	88,809
Fixed maturity available-for-sale securities, at estimated fair value	$ 87,900	$ 93,534