Amortized Cost and Estimated Fair Value of Fixed Maturity Available-for-Sale Securities by Investment Grade (Detail) - USD ($) $ in Thousands	Sep. 30, 2017	Dec. 31, 2016
Schedule of Available-for-sale Securities [Line Items]
Fixed maturity available-for-sale securities, amortized cost	$ 1,457,131	$ 1,383,787
Fixed maturity available-for-sale securities, at estimated fair value	1,585,900	1,487,037
Investment grade
Schedule of Available-for-sale Securities [Line Items]
Fixed maturity available-for-sale securities, amortized cost	1,379,519	1,294,978
Fixed maturity available-for-sale securities, at estimated fair value	1,498,855	1,393,503
Below investment grade
Schedule of Available-for-sale Securities [Line Items]
Fixed maturity available-for-sale securities, amortized cost	77,612	88,809
Fixed maturity available-for-sale securities, at estimated fair value	$ 87,045	$ 93,534