Amortized Cost and Estimated Fair Value of Fixed Maturity Available-for-Sale Securities by Contractual Maturities (Detail) - USD ($) $ in Thousands	Jun. 30, 2017	Dec. 31, 2016
Amortized cost of fixed maturity AFS securities
Due in one year or less	$ 78,733	$ 56,748
Due after one year through five years	450,657	523,815
Due after five years through ten years	118,259	95,744
Due after ten years	497,496	485,720
Available-for-sale Securities, Debt Maturities, Single Maturity Date, Amortized Cost Basis, Total	1,145,145	1,162,027
Mortgage-backed securities and other asset-backed securities	202,773	221,760
Fixed maturity available-for-sale securities, amortized cost	1,347,918	1,383,787
Estimated fair value of fixed maturity AFS securities
Due in one year or less	80,116	57,489
Due after one year through five years	477,698	555,221
Due after five years through ten years	124,484	99,697
Due after ten years	582,290	549,354
Available-for-sale Securities, Debt Maturities, Single Maturity Date, Total	1,264,588	1,261,761
Mortgage-backed securities and other asset-backed securities	210,238	225,276
Total fixed maturity AFS securities	$ 1,474,826	$ 1,487,037