Amortized Cost and Estimated Fair Value of Fixed Maturity Available-for-Sale Securities by Contractual Maturities (Detail) - USD ($) $ in Thousands	Sep. 30, 2018	Dec. 31, 2017
Amortized cost of fixed maturity AFS securities
Due in one year or less	$ 122,414	$ 73,570
Due after one year through five years	325,203	466,858
Due after five years through ten years	125,951	134,939
Due after ten years	454,995	539,151
Available-for-sale Securities, Debt Maturities, Single Maturity Date, Total	1,028,563	1,214,518
Mortgage-backed securities and other asset-backed securities	145,174	203,610
Fixed maturity available-for-sale securities, amortized cost	1,173,737	1,418,128
Estimated fair value of fixed maturity AFS securities
Due in one year or less	123,874	74,686
Due after one year through five years	329,235	484,576
Due after five years through ten years	124,188	139,174
Due after ten years	495,337	638,558
Available-for-sale Securities, Debt Maturities, Single Maturity Date, Total	1,072,634	1,336,994
Mortgage-backed securities and other asset-backed securities	151,886	211,644
Total fixed maturity AFS securities	$ 1,224,520	$ 1,548,638