Amortized Cost and Estimated Fair Value of Fixed Maturity Available-for-Sale Securities by Contractual Maturities (Detail) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Amortized cost of fixed maturity AFS securities
Due in one year or less	$ 73,570	$ 56,748
Due after one year through five years	466,858	523,815
Due after five years through ten years	134,939	95,744
Due after ten years	539,151	485,720
Available-for-sale Securities, Debt Maturities, Single Maturity Date, Total	1,214,518	1,162,027
Mortgage-backed securities and other asset-backed securities	203,610	221,760
Fixed maturity available-for-sale securities, amortized cost	1,418,128	1,383,787
Estimated fair value of fixed maturity AFS securities
Due in one year or less	74,686	57,489
Due after one year through five years	484,576	555,221
Due after five years through ten years	139,174	99,697
Due after ten years	638,558	549,354
Available-for-sale Securities, Debt Maturities, Single Maturity Date, Total	1,336,994	1,261,761
Mortgage-backed securities and other asset-backed securities	211,644	225,276
Total fixed maturity AFS securities	$ 1,548,638	$ 1,487,037