Summary of Amortized Cost and Estimated Fair Value Through Fixed Maturities (Detail) - USD ($) $ in Thousands	Sep. 30, 2016	Dec. 31, 2015
Schedule of Available-for-sale Securities [Line Items]
Due in one year or less, Amortized Cost	$ 81,811
Due after one year through five years, Amortized Cost	633,977
Due after five years through ten years, Amortized Cost	47,779
Due after ten years through fifteen years, Amortized Cost	0
Due after fifteen years, Amortized Cost	3,441
Fixed maturities, amortized cost	1,297,465	$ 1,308,333
Due in one year or less, Estimated Fair value	82,130
Due after one year through five years, Estimated Fair value	642,829
Due after five years through ten years, Estimated Fair value	48,727
Due after ten years through fifteen years, Estimated Fair value	0
Due after fifteen years, Estimated Fair value	3,469
Fixed Maturities, estimated fair value	1,310,957	$ 1,306,149
Mortgage Backed Securities
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	104,222
Estimated Fair value	106,949
Asset-backed Securities
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	250,766
Estimated Fair value	252,200
Commercial Mortgage-Backed Securities
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	175,469
Estimated Fair value	$ 174,653