Amortized Cost and Estimated Fair Value of Fixed Maturity Available-for-Sale Securities by Contractual Maturities (Detail) - USD ($) $ in Thousands	Jun. 30, 2016	Dec. 31, 2015
Amortized cost of fixed maturity AFS securities
Amortized cost of fixed maturity AFS securities due in one year or less	$ 72,334	$ 64,114
Amortized cost of fixed maturity AFS securities due after one year through five years	535,068	564,707
Amortized cost of fixed maturity AFS securities due after five years through ten years	151,972	230,858
Amortized cost of fixed maturity AFS securities due after ten years	486,591	465,410
Available-for-sale Securities, Debt Maturities, Single Maturity Date, Amortized Cost Basis, Total	1,245,965	1,325,089
Amortized cost of mortgage-backed securities and other asset-backed securities	317,808	245,604
Fixed maturity available-for-sale securities, amortized cost	1,563,773	1,570,693
Estimated fair value of fixed maturity AFS securities
Estimated fair value of fixed maturity AFS securities due in one year or less	72,911	65,086
Estimated fair value of fixed maturity AFS securities due after one year through five years	577,160	599,965
Estimated fair value of fixed maturity AFS securities due after five years through ten years	163,110	230,815
Estimated fair value of fixed maturity AFS securities due after ten years	604,437	515,868
Available-for-sale Securities, Debt Maturities, Single Maturity Date, Total	1,417,618	1,411,734
Estimated fair value of mortgage-backed securities and other asset-backed securities	327,454	253,603
Fixed maturity available-for-sale securities, at estimated fair value	$ 1,745,072	$ 1,665,337