Investments - Scheduled Maturities of Company's Fixed Maturity Securities Based on their Fair Values (Detail) - USD ($) $ in Thousands	Sep. 30, 2017	Dec. 31, 2016
Schedule of Available-for-sale Securities [Line Items]
Securities with Unrealized Losses, Total	$ 83,472	$ 86,000
Fixed Maturities
Schedule of Available-for-sale Securities [Line Items]
Securities with Unrealized Gains, One year or less	1,571
Securities with Unrealized Gains, After one through five years	12,245
Securities with Unrealized Gains, After five through ten years	2,841
Securities with Unrealized Gains, After ten years	4,733
Securities with Unrealized Gains, No single maturity date	14,099
Securities with Unrealized Gains, Total	35,489
Securities with Unrealized Losses, After one through five years	20,915
Securities with Unrealized Losses, After five through ten years	27,431
Securities with Unrealized Losses, After ten years	3,051
Securities with Unrealized Losses, No single maturity date	30,465
Securities with Unrealized Losses, Total	81,862
All Fixed Maturity Securities, One year or less	1,571
All Fixed Maturity Securities, After one through five years	33,160
All Fixed Maturity Securities, After five through ten years	30,272
All Fixed Maturity Securities, After ten years	7,784
All Fixed Maturity Securities, No single maturity date	44,564
All Fixed Maturity Securities, Total	$ 117,351