Amortized Cost and Estimated Fair Value of Fixed Maturity Available-for-Sale Securities by Contractual Maturities (Detail) - USD ($) $ in Thousands	Mar. 31, 2018	Dec. 31, 2017
Amortized cost of fixed maturity AFS securities
Due in one year or less	$ 78,457	$ 73,570
Due after one year through five years	418,785	466,858
Due after five years through ten years	124,237	134,939
Due after ten years	448,414	539,151
Available-for-sale Securities, Debt Maturities, Single Maturity Date, Total	1,069,893	1,214,518
Mortgage-backed securities and other asset-backed securities	149,550	203,610
Fixed maturity available-for-sale securities, amortized cost	1,219,443	1,418,128
Estimated fair value of fixed maturity AFS securities
Due in one year or less	79,445	74,686
Due after one year through five years	428,159	484,576
Due after five years through ten years	124,425	139,174
Due after ten years	507,862	638,558
Available-for-sale Securities, Debt Maturities, Single Maturity Date, Total	1,139,891	1,336,994
Mortgage-backed securities and other asset-backed securities	154,430	211,644
Total fixed maturity AFS securities	$ 1,294,321	$ 1,548,638