Investments - Scheduled Maturities of Company's Fixed Maturity Securities Based on their Fair Values (Detail) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Schedule of Available-for-sale Securities [Line Items]
Securities with Unrealized Losses, Total	$ 89,478	$ 86,000
Fixed Maturities
Schedule of Available-for-sale Securities [Line Items]
Securities with Unrealized Gains, One year or less	2,219
Securities with Unrealized Gains, After one through five years	4,798
Securities with Unrealized Gains, After five through ten years	1,750
Securities with Unrealized Gains, After ten years	4,715
Securities with Unrealized Gains, No single maturity date	12,991
Securities with Unrealized Gains, Total	26,473
Securities with Unrealized Losses, After one through five years	26,818
Securities with Unrealized Losses, After five through ten years	28,372
Securities with Unrealized Losses, After ten years	3,052
Securities with Unrealized Losses, No single maturity date	29,187
Securities with Unrealized Losses, Total	87,429
All Fixed Maturity Securities, One year or less	2,219
All Fixed Maturity Securities, After one through five years	31,616
All Fixed Maturity Securities, After five through ten years	30,122
All Fixed Maturity Securities, After ten years	7,767
All Fixed Maturity Securities, No single maturity date	42,178
All Fixed Maturity Securities, Total	$ 113,902