Amortized Cost and Estimated Fair Value of Fixed Maturity Available-for-Sale Securities by Contractual Maturities (Detail) - USD ($) $ in Thousands	Sep. 30, 2017	Dec. 31, 2016
Amortized cost of fixed maturity AFS securities
Due in one year or less	$ 73,627	$ 56,748
Due after one year through five years	481,655	523,815
Due after five years through ten years	141,297	95,744
Due after ten years	532,027	485,720
Available-for-sale Securities, Debt Maturities, Single Maturity Date, Amortized Cost Basis, Total	1,228,606	1,162,027
Mortgage-backed securities and other asset-backed securities	228,525	221,760
Fixed maturity available-for-sale securities, amortized cost	1,457,131	1,383,787
Estimated fair value of fixed maturity AFS securities
Due in one year or less	75,222	57,489
Due after one year through five years	506,333	555,221
Due after five years through ten years	147,232	99,697
Due after ten years	619,466	549,354
Available-for-sale Securities, Debt Maturities, Single Maturity Date, Total	1,348,253	1,261,761
Mortgage-backed securities and other asset-backed securities	237,647	225,276
Total fixed maturity AFS securities	$ 1,585,900	$ 1,487,037