Summary of Amortized Cost and Estimated Fair Value Through Fixed Maturities (Detail) - USD ($) $ in Thousands	Dec. 31, 2016	Dec. 31, 2015
Schedule of Available-for-sale Securities [Line Items]
Due in one year or less, Amortized Cost	$ 80,840
Due in one year through five years, Amortized Cost	623,678
Due in five years through ten years, Amortized Cost	20,356
Due in ten years through fifteen years, Amortized Cost	3,245
Due after fifteen years, Amortized Cost	6,378
Fixed maturities, amortized cost	1,241,339	$ 1,308,333
Due in one year or less, Estimated Fair value	80,982
Due in one year through five years, Estimated Fair value	622,926
Due in five years through ten years, Estimated Fair value	20,770
Due in ten years through fifteen years, Estimated Fair value	3,252
Due after fifteen years, Estimated Fair value	6,450
Fixed Maturities, estimated fair value	1,240,031	$ 1,306,149
Mortgage Backed Securities
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	88,131
Estimated Fair value	88,468
Asset-backed Securities
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	233,890
Estimated Fair value	233,991
Commercial Mortgage-Backed Securities
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	184,821
Estimated Fair value	$ 183,192