Amortized Cost and Estimated Fair Value of Fixed Maturity Available-for-Sale Securities by Investment Grade (Detail) - USD ($) $ in Thousands	Jun. 30, 2016	Dec. 31, 2015
Schedule of Available-for-sale Securities [Line Items]
Fixed maturity available-for-sale securities, amortized cost	$ 1,563,773	$ 1,570,693
Fixed maturity available-for-sale securities, at estimated fair value	1,745,072	1,665,337
Investment grade
Schedule of Available-for-sale Securities [Line Items]
Fixed maturity available-for-sale securities, amortized cost	1,461,611	1,467,677
Fixed maturity available-for-sale securities, at estimated fair value	1,642,016	1,565,053
Below investment grade
Schedule of Available-for-sale Securities [Line Items]
Fixed maturity available-for-sale securities, amortized cost	102,162	103,016
Fixed maturity available-for-sale securities, at estimated fair value	$ 103,056	$ 100,284